Mendel's Impact on Genetics and Heredity

Hey everyone, Dr. D here, and in this video we are going to be covering chapter 14 from our Campbell's 12th edition biology textbook. And in this chapter we'll be covering Mendel and Heredity. So let's go ahead and get started. Dr. D, Dr. D, explain stuff. All right, welcome back. Let's go ahead and get started with chapter 14 covering Mendel and heredity. Mendel is known as the father of modern genetics and that's because Mendel discovered the basic principles of heredity by breeding garden peas. These pea plants are called Pysum sativa and they're amazing. They helped us to crack the genetic code and to understand how heredity works. Isn't that neat? before Mendel and his experiments, along with a handful of other researchers like T.A. Knight, we really didn't understand heredity very well. This was in the 1800s. And in the early to mid 1800s, humanity had no clue how heredity worked. We didn't know how traits are passed on from generation to generation. Like you have brown eyes and, you know, your child might have brown eyes, but we didn't. really know how this heredity worked. In fact, at the time, blending hypothesis was a big idea in how heredity works. And that is that your children are a blend of you. And Mendel really helped to dispel and disprove this concept of blending. And so let's delve into his findings and how he used this really... humble pea plant in order to crack some of humanity's greatest mysteries regarding heredity. Mendel was a very interesting character in the history of science. Mendel actually went to the University of Vienna and studied science under some brilliant minds, including Christian Doppler, who taught him how to do a proper scientific study, as well as famous botanists of the time as well. So he learned about botany. He learned about the pea plant. He learned about how to conduct a proper scientific study. So he was always interested in science, but he went on to become a monk. And then later on, in the very abbey gardens of the monastery he worked at, he actually set up genetic crosses using these pea plants, different variety of pea plants. What a brilliant and interesting person combining both. His passion for, you know, religion as well as his passion for science, setting up these genetic studies, the results of which were groundbreaking and earned him the name the father of modern genetics. He was instrumental at discovering how heredity worked. At the time, keep in mind, this was the mid to late 1800s. People didn't even know about the chromosomes or genes or how important genes are. And yet he was able to deduce using the humble pea plant how heredity works, you know, these concepts of passing on your traits to your offspring and how those traits manifest in your... progeny. Isn't that neat? And let's see what his contributions were. Let's discover and learn about his contributions and how he helped to understand how heredity works. One reason why the pea plant was a great choice for Mendel and others to use to understand heredity was that so many different varieties were available. And these varieties had different... what? mendel called characters characters are heritable features that vary among the individuals so for example flower color if we're talking you know the pea plant was a flowering plant and flower color would be an example of a character but you know that how there's different versions of that character like you could have purple flower color you could have a variation which is a white flower color so the variants were called Does that make sense? The character was Flower Color, and to Mendel, the traits were the variations. for a character such as purple or white. I hope that makes sense. Again, character would be flower color. Purple or white would be traits. Just like, for example, Mendel would say eye color is a character. Blue eyes are a trait. Brown eyes are a trait. So brown and blue are traits of the character eye color. I hope that makes sense. So let's remember this terminology as we move forward. Speaking of variety. No kidding, look at this. There were so many different variety of these pea plants. You could look at flower color, purple versus white. So again, flower color would be the characteristic, purple and white being the traits. You could look at seed color characteristic, yellow seeds versus green seeds being the traits, seed shape being the characteristic, round versus wrinkly. Those are the traits. Pod color. You could study pod color. There were green variants and yellow variants or traits. Pod shape, inflated pods, constricted pods, flower position, stem length. Look how much variation existed in these pisum, sativum, pea plants. So much variety. Great when you're studying heredity because heredity is the study of how these traits are passed on to the offspring. That's great if you have all this variety to study. Other advantages of using the pea plant? Short generation time, you know these things grow quite quickly. Large numbers of offspring because each pea can be planted. You can think of each pea as an embryo and each pea can be planted and gives rise to a whole new plant so you can have large number of offspring. And this is really interesting. Mating could be controlled. Plants could be allowed to either self-pollinate, that means fertilize themselves, or could be cross-pollinated. This means that one plant could be used to fertilize a separate plant. Let me explain this a little bit more detail at the board. The pea plant was a brilliant model organism to use because unlike... animals which require a male and a female to breed to have offspring. The pea plant could do one better and let me explain why. If you have a pea plant over here, here I have a pea plant with a purple flower. This pea plant you could cross it with another pea plant right as you would expect. You could take pollen which pollen contains the sperm. You could take pollen from this pea plant and then inseminate this other pea plant with that pollen and you could have offspring, right? However, the brilliance in using these pea plants in heredity studies is that not only can these flowering plants produce pollen which contains the sperm, but they also produce the eggs as well. So a single flower produces the sperm and the egg, which means effectively you don't need to have another plant. You could what's called self-fertilize these plants. Does that make sense? Just allow the sperm to travel a short distance and fertilize the eggs. inside of the flower. So the flower again, it contains the sperm and the egg. If you can make sperm and egg you could fertilize yourself and this is known as self-fertilization. And then you could study the offspring. You could study the offspring of the self-fertilization. So this is why the plant was amazing. You could either cross-fertilize with a separate plant or you could self- fertilize that plant with itself that's just something you can't do with animals such as mice Here's how it all works. This is a flower such as those found on Pysum sativa, the pea plant, the flowering pea plant. This is a purple flower obviously. Now the flower can contain anthers, which are the male portion of the flower, and the anther can form pollen at the tip of these structures, these filamentous structures. And remember the pollen contains the sperm. Now the flower also possesses the carpal, the female portion of the flower, which contains these eggs. All the pollen has to do is travel from the tip of the anther to stick to the stigma, travel down the style, and fertilize all these little eggs. And once those little eggs are fertilized, those fertilized eggs will be the ones that will be the most important. eggs become the peas, the peas in the pea pod. Isn't that neat? So you can think of a pea as a embryo, a small fertilized embryo, right? And that embryo, when you plant that pea, it's like planting an embryo, which becomes the new plant. It's a seed. It becomes the new plant. So that pea could give rise to another purple flower plant. Does that make sense? So What you do if you want to cross-fertilize a plant, you simply take a paintbrush and you pick up some pollen from one flower and you deposit on another flower, right? So that's how you cross-fertilize. Now to prevent self-fertilization, what Mendel would do is cut off the anther from a immature flower, right? Wait for... you know, the flower to form. But before the flower is mature, cut the anther off, making it impossible for self-fertilization to occur, and then to inseminate that flower, you know, that denuded flower with pollen from another plant. Does that make sense? So this would be what you need to do to cross-fertilize. If you wanted to self-fertilize, well then don't cut the anther off. Leave the... leave the anther on and if you leave the anther on you could allow for self-fertilization to occur i hope this makes sense so again these are amazing plants to use amazing model organisms because you can either self-fertilize and see the offspring or cross-fertilize to see offspring all right let me share with you something so important this was a total mystery at the beginning of this quest to understand heredity for for Mendel and other researchers such as T.A. Knight, this is what started it all. What they tried was to self-fertilize these plants and they noticed something very bizarre when they did. So if they had a purple flower plant and they self-fertilized this purple flower plant, what would you expect to see? What would you expect the offspring to look like if you took a purple flower plant? and you self-fertilized it, you used its own pollen to fertilize its own eggs using just this one plant, what would you expect all the offspring to look like? That's right, Wicket. You would expect them to all be purple, right? And that's what he would see for this plant. Like this plant, he would self-fertilize it and 100% of all the offspring would be purple flower. purple flower Plants and that makes perfect sense right your purple flower you self fertilized yourself You should have purple flower offspring and that's what he saw that that makes perfect sense But here's where it got weird you ready now he would mosey over to this purple flower plant You know same pie some sad of them same species and he would do the same thing. He would say okay Purple flower plant. I'm going to use its pollen to fertilize its own eggs. Great. And this time something weird would happen. This time he had mostly purple, but guess what? Some white. Some white flower plants would emerge as well. So in this case some or most of his offspring were purple. And some were white. That's really weird, right? I mean, this plant, if you self-fertilize it, it will always give you purple flower offspring. This plant right next to it, you know, he would self-fertilize it and it would give you purple and sometimes white offspring. Is that not bizarre? That was the big mystery, right? That was, that was what really got them thinking. And this is, this is something very important because there's genius behind this. This was the genius. You guys ready? What Mendel and others like Knight would call this plant is a true breeder. And we need to know that this he called a true breeder. Okay? And the reason he called it a true breeder is because when you self-fertilize it with itself, it always looks like itself. Its offspring always look like itself. It doesn't matter how many offspring you make. This one, on the other hand, is that a true breeder? No, because sometimes white flower plants would emerge. It's like, where did that come from? Right. So he called these ones non true breeders, non TBs for short, non true breeders. OK, so this is very important to understand because of this next step. Oh, and before I forget one more very strange thing. He also had white flower variants of these. pisum sadova plants with white flowers. And you know what's strange? When he would self-fertilize these white flowers, they would always have white flower offspring, but he could never find a white flower plant that was not a true breeder. Isn't that strange? So the mystery thickens, right? The plot thickens. With the purple flower plant, he noticed some purple flower plants were true breeders, meaning they would always look like themselves when you self-fertilize. And some of his purple flower plants were non-true breeders. So of course this led to a cross where Mendel wanted to know just what happens when you cross a purple flower plant with a white flower plant. But this was the genius part. You ready for the genius part? He didn't take just any purple flower plant. What do you think he took? A true breeder, TB for short, true breeder purple flower plant. And he crossed it with a true breeder white flower plant. Does that make sense? By the way, like I said before, every white flower plant he found was a true breeder. So that wasn't hard to find a true breeder white flower plant. It was the purple flower plants that were sometimes true breeders and other times not true breeders. Right. So this was the genius of it all. They made sure Mendel made sure to start with a true breeder purple flower plant. and cross it with a true breeder white flower plant. And that is what made this study work. If he had picked a non-true breeder purple, then that would have really not made much sense. Okay, so does that make sense? So at this point, you can do what's called a hybrid hybridization or a hybrid cross. And so Mendel did a mono hybrid cross with this. When he took sperm. or pollen from one flower and used it to fertilize the eggs on the next flower. So let's see how that turned out. Alright, so to start out, Mendel did his famous mono-hybrid cross experiment. What does the mono-hybrid cross mean? Mono refers to a single characteristic that he was studying. In this case, the characteristic was flower color. And there are two traits for flower color, purple and white. Remember, I previously mentioned that Purple flower plants could either be true breeders or non true breeders. Here was the genius of his experiment. He always started with true breeders, right? So true breeder purple crossed with a true breeder white. And that's what a hybridization refers to. Hybridization occurs between two true breeders. So when you cross a true breeder purple with a true breeder white, this is an example of a hybridization. And it's called a monohybrid again because it's a single trait, or I should say a single characteristic that he was studying. Now, what happened? During this cross-fertilization, pollen from one plant is used to fertilize the other flower, the other plant. And then we could study the offspring. Let's see what he saw. So check it out. 100% of the offspring of this hybrid cross were purple flower plants. He could not find a single white flower plant offspring. And that is amazing. So that was a very curious observation. And By the way, it already negated the entire concept of the blending hypothesis. Remember the whole blending hypothesis? People thought that your offspring were a blend of you and your spouse. And so here we see there's no blending going on. These flowers were not purpley white. They were just as purple as this purple parent right here. There wasn't a hint of white flower color. in this purple offspring. By the way, at this point I want to show you some terminology. The original parents are called the parent generation, that makes sense, or sometimes it's the P generation or even the P1 generation. These are the parents. The offspring are called the filial generation 1, or the F1 generation for short. Filial refers to your son, you know, or at least in Latin. So the F1 generation are the offspring of the parent generation. So when I say I'm referring to the F1 generation, I'm talking about the offspring of the original parents, the true breeder parents. So this is the offspring of two true breeders, and it looks like the white information is missing. So at this point. You know, Mendel and others had to scratch their head, you know. Where did the white information go? Think about that. Where did the white information go? Is it missing? Did it never get any white information? Did it get destroyed? Did the white information get destroyed? What happened to this white trait? Is it in there somewhere? Is the white trait in this organism? but it's being masked somehow. You know, all of these were equally possible, you know, outcomes at the time, until they did the experiment, right? So what they did, what Mendel did, this was the beauty in using plants as the model organism here, was that he took this plant and he self-fertilized these plants, self-fertilized. Again, take the pollen from that flower, Used it to fertilize that very flower on that very plant, self-fertilizing these to see the outcome. And let's take a look at the outcome. All right, take a look at this. When this plant was self-fertilized, it resulted in offspring that most of the time were purple. But guess what? White made a comeback. And Mendel knew enough, you know, with his studies at University of Vienna and all of his training as a scientist, he knew enough to do this experiment hundreds of times. And so he was able to establish that not only does white color come back, it comes back. back one quarter of the time. So what Mendel saw was that of the offspring of this self fertilization, three are purple for every one white and he called this the three to one ratio, the three to one phenotypic ratio. Let me define that term real quick, phenotype. The phenotype is what we see, it's the trait we see, we see purple and white. So what would you say in this F2 generation? That's right, the grandchildren are called the F2 generation. And what do we see? We see three purples for every one white. And so Mendel saw a three to one phenotypic ratio, three purples for every one white in the grandchildren of the parent generation. When you self-fertilize the hybrid, This is why it's, this is, this is what we're talking about. This is the hybrid, the hybrid self-fertilized, and in its offspring you see a three to one purple to white ratio. Now white came back, white came back, so what does that tell you? That tells you that the white information was still in this purple flower somewhere, in this plant somewhere. It was just being masked. It wasn't missing. The white information wasn't missing. It didn't get destroyed, it was in there somewhere. It was just being masked. So the purple information was somehow masking the white information. And somehow in the F2 generation, it presented itself again. So this led him to a very important finding. Let's talk about that. What this showed Mendel was that each individual has information from both parents. Okay, so this individual, not only does this individual have purple color information from the purple parent, so it has the purple trait from the purple parent, but this individual also inherited white trait information from the other parent. So What's really interesting was that although the characteristic was flower color, each individual possesses two traits for every characteristic. So if the characteristic is flower color, I can possess purple information and white information both. I can possess both. So... So each individual possesses two traits for every one characteristic. And not only do you possess them, one can be dominant to the other. In this case, purple trait information was masking or dominant to the white information, which is called recessive. I hope you know where we're going with this. By the way, just to give you some foreshadowing, nowadays we don't use the terms characteristic anymore. Instead of characteristic, we say gene, right? And we don't use the term trait so much. We use the term allele. Okay, so if a characteristic was, you know, a general characteristic like flower color, now we call that flower gene, the flower color gene. Okay, and... Different versions of that gene are called alleles. So purple would be an allele, white would be an allele. Of what? Of the flower color gene. Mendel referred to these as characteristics instead of genes, and as traits instead of alleles, but now we know the current terminology. And that's really interesting, right? So just like you have two parents, you know, and, you know, The parents have, you know, you inherited eye color from each of your parents. Well, you could have inherited blue eye color allele from your mom and brown eye color allele from your dad, right? So you have two alleles for that gene. You inherited blue eye color allele from your mom. You inherited brown eye color allele from your dad. One. is dominant to the other, therefore you may have brown eyes, right? But that doesn't mean you don't have the blue allele information inside, it's just being masked. So just to give you some foreshadowing of what you're seeing here, this flower... has the purple allele from that parent and the white allele from that parent. Then when this plant self fertilized, its offspring have different outcomes. This one right here shows the white allele outcome, the white flower, and the rest of the three show the dominant color. So they're called the purple dominance. And three dominants appear for every one recessive. So we say, again, the monohybrid cross results in a three-to-one phenotypic ratio of dominance to recessives every time. And so, again, the big takeaway here is that each individual inherits one allele from, you know, each parent, one allele. from each parent, so each individual has two alleles for every one gene, two traits for every one characteristic, and you pass on one of those two to your offspring. Does that make sense? For example, this plant here has a purple, let's call it capital P for purple, capital P allele refers to the purple allele. And we'll do the lowercase p to refer to the white allele, right? So the purple allele is from this purple parent, and the lower p allele, the white allele, is from this parent. Does that make sense? So each individual has two alleles for every characteristic, for every gene. If that's true, let me ask you this. If that's true... Shouldn't this parent also have two alleles for the flower color characteristic or two alleles for that gene? That's true. But what would both alleles say? If this is a true breeder, what do you think both of these alleles would say? Can you beat Wicket? Obviously, exactly right, Wicket. This parent does have two alleles, but guess what? They both say purple, and that's why it's a true breeder. Isn't that neat? The fact... that this has a small p, a white allele, makes it a non-true breeder. Now we know what non-true breeders are, you guys. Remember, this was baffling early on to Mendel and others, right? Like, what in the world are these non-true breeders? How come some of my plants, when you self-fertilize them, they only give offspring that look like them? And how come sometimes when I self-fertilize them, I get a different outcome? Well, it's because, is it a Non-true breeder, you know, does it have two different alleles? Or is it a true breeder? Does it have two of the same allele? Isn't that interesting? You're getting it now? Hopefully we're making the concepts come together. And then what should this parent be? That's right, two little peas, right? Now let me ask you this. Was Mendel ever able to find a white flower plant that was a non-true breeder? That's right, Wicket. He wasn't. And do you see now why? Do you see why he couldn't find a non-true breeder white? Well, it's because you can't have anything other than little p, little p, and be white color, because if you have a big P, you're going to be purple. So the only way to show the white trait is to have two white alleles, have two white traits, right? Does that make sense? So the true breeder whites... have to be little p, little p. And so there's no such thing as a non-true breeder white flower because that's the recessive gene. If there was another allele, you'd show that other trait. You wouldn't show the white trait. Isn't that neat? So again, true breeder purple, big P, big P. True breeder white, little p, little p. Look what this, look what happened. Mendel said we have two alleles for every. characteristics, so we, or gene, right? See, two alleles, two alleles, two alleles, but do you pass on both of them to your offspring? Did this parent pass on both of these peas? Did this parent pass on both of their peas or just one? Just one, right? So that's another important thing that Mendel said, was that you possess two alleles, but you pass on one or the other. to your offspring. Does that make sense? So this parent passed on one big P allele, one purple allele. This parent passed on one little p, one white allele, and so this individual, this hybrid inherited one allele from one parent and the other allele from the other parent, and so this led to what Mendel called the principle of segregation of alleles, and this was the famous outcome of this monohybrid experiment. So if we ask you what was the main takeaway? What was the main point to doing this monohybrid cross? What did it teach us as humans? It taught us the principle of segregation of alleles, that each individual has two alleles for every one gene, but you pass on one or the other at random to your offspring. Your offspring only get one or the other. Does that make sense? And those come together randomly in the fertilized offspring. Isn't that neat? Again, remember the principle of segregation of alleles suggests that each individual has two alleles, and those alleles segregate into the progeny. So if that's true, let's see if we can figure this out. This parent, again, was big P, big P, two purple alleles. This parent was little p, little p. Remember, the lower case refers to the recessive allele. So in this case, little p is the white allele. Now, this parent, let's say this parent made the sperm. Let me ask you this. What did we learn about meiosis in the previous chapter? Are sperm cells haploid or diploid? You got it, Wicket. Sperm cells are haploid. Gametes are haploid. Do you know the reason? Do you know the reason why each individual has two alleles? Well, it's because one allele is from one homologue, one chromosome from one parent. So let's say this big P is on chromosome one from its mom, from the egg, and the other big P is on its chromosome one from its dad, you know, from the sperm. So the two Ps are on homologous chromosomes is what I'm trying to say. Do sperm cells get both homologues? Do they get both chromosomes from both parents, or do they just get one? Do you remember that from meiosis? In meiosis, the sperm are haploid. This means that only one of these two, it could be either one, right? But only one of these two alleles gets packaged into any one sperm because the sperm are haploid. If the sperm got both P's, then the sperm would be diploid. Does that make sense? Hopefully that makes better sense. So what could be in the egg then? The egg gets either this little pea or this little pea, but either way the egg gets a little pea allele, the sperm gets the big pea allele, and when the sperm fertilizes the egg, what is the result of that sperm fertilizing this egg? That's right. The zygote has to be big pea, little pea, and that's what this flower is. This plant is a hybrid. It is a non-true breeder. It's got a big P and a little p. And now this is a good time to introduce a term that you need to know. It's called genotype. Okay, genotype refers to the alleles that you possess. It's your allele set. So when I say what's the genotype of this plant, You could tell me it's big P, little p. It has a purple allele and a white allele. That's why it's a non-true breeder. But if I were to ask you, what's the phenotype of this plant? Well, then you have to use your eyes, right? It's purple. Does that make sense? Are you following along? So genotype refers to what alleles I possess, like big P, little p, little p, little p, big P, big P. These are genotypes. phenotypes are what I actually see. I see purple. I see white. I see purple. You see what I'm saying? All right. So what did we do next? Do you recall? The next thing we did, didn't we self-fertilize this F1 offspring? The F1 offspring was self-fertilized to give the F2 offspring, right? So what is it? If we're both parents, if we're the mom and the dad, if we're both parents, isn't that like... Taking a big P, little p, and crossing it with yourself, a big P, little p, does that make sense, right? Wouldn't that be the same as a cross between two non-true breeders, I guess, if you're both parents? So if that's true... You know, what kind of sperm could you make, right? What kind of, let's say the sperm, right? What could be in the sperm, okay? Let's see if you can beat wicket. Yeah, sure, why not wicket, yeah. A big P, so a purple allele could be in a sperm. Could a little P be in the same sperm? No, because that would make the sperm diploid. So that means the other sperm can get what? What could be in the other sperm? That's right. The little p could be packaged into one sperm. The big p could be packaged into the other sperm. This is what we're talking about, principle of segregation of alleles. You have both alleles, but they segregate into the gametes, and then they recombine into the progeny upon fertilization. So here, what about eggs? Let's say this is the mom, the eggs, you know, this is the mom and the dad in one. But what could be in the eggs? That's right, a big p egg. Or could you make a little p egg? Sure. Does that make sense? So you can make both gamete types, right? Now, let me ask you this. Let me ask you this. Could this sperm with a big P fertilize this egg with a little P, with a big P as well? Could that big P sperm fertilize this big P egg? Sure. And what would the result be? What would the genotype of the outcome be? Big P, big... P, right. Big P, big P. Great. Now let me ask you this. Would it be equally possible for this big P sperm to fertilize the little P egg? Sure, why not, right? So that would be big P, little P. Sure, okay. And would it be possible for this little P sperm to fertilize the big P egg? Sure. And by the way, always put the capital letter first if you can when you're pairing these up. Okay. And so, and what's the final combo? What's the final fertilization event? What if the little p sperm fertilized the little p egg? Is that possible? Why not? Why not? So what would that give you? Little p, little p. And let me ask you this. Would that series of random events explain the whole thing? Mendel's observations in the F2 generation? Would it explain your three to one phenotypic ratio? It would, look! You would get three purple color plants because remember you just need one capital P to show the purple color and you would end up with only one out of these four events, one white color offspring. Does that make sense? Only one little p little p. So we exactly showed it. So guess what? Doesn't that mean, let me ask you this, let me ask you this, doesn't that mean that the first plant should be a true breeder? Like what if I self-fertilized this guy? What should I see in the offspring? Exactly right, Wicket. This plant should be a true breeder. If I self-fertilize this plant, it should only give me purple offspring. But what about these two plants? What if I self-fertilize these two plants? Ah, there you go. It's like self-fertilizing this guy. Isn't that the same? So I should see a 3 to 1 ratio of purples and whites if I were to self-fertilize these, right? And then what about this one? What should that give me? If I self-fertilize this white flower plant, I should only see white flower offspring. So do you want to see what Mendel saw? Let's check it out. All right, look at this you guys. This is exactly what Mendel did. He went that extra step and did exactly what we said. He self-fertilized this and saw the offspring. What did the offspring show? It was a true breeder. That's right. This was a big pea, big pea. Does that make sense? And what about these two? What did he show? That's right. The white flower came back and it came back in a three to one ratio. So again, these are big pea, little peas. Okay, and what did he show with the white flower when he self-fertilized it? That's right. It is a true breeder, little p, little p. And so let's wrap this up. Let me tell you something important, right? What was the phenotypic ratio? And by the way, anytime you're asked on a test, what's the phenotypic ratio or the genotypic ratio? You need to refer to the F2 generation. They're talking about the F2 generation. Okay, now what was the phenotypic ratio, the phenotypic ratio of the monohybrid cross? What was it? Can you beat Wicket? Perfect. Three dominants to every one recessive. You see my three purples for every one white? Three dominants to every one recessive. My phenotypic ratio for the monohybrid cross is always three to one. Now, let me ask you this. What's my genotypic ratio? So what does that refer to? That means the types of genotypes, right? The ratios of genotypes in the F2 generation. So let me write this down. The genotypic ratio. So let me ask you this. Here's how you figure out the genotypic ratio. You go to the F2 generation and you ask yourself, what do I have? Look at this. What's the genotype of this offspring? Isn't it big P, big P? Okay, that means dominant, dominant. How many dominant dominants do we have? How many dominant dominants do we have? True breeder dominants. That's right, I have one, right? Now let me ask you this. How many non-true breeders do I have? That means how many genotypes do I have where it's dominant recessive, right? Big P, little p, right? How many of those do we have? Two, right? Two. So I have one big P, big P, two big P, little p's, and now how many little p, little p's do I have? I only have one. So what is my genotypic ratio for the monohybrid cross? It is 1 to 2 to 1. One big P, big P for every two big P, little p's for every one little p, little p. It's a 1 to 2 to 1 genotypic ratio, which gives me my 3 to 1 phenotypic ratio. And that is exactly what you need to know for Mendel's famous monohybrid cross. All right, so in the dihybrid cross, Mendel was studying two characteristics at the same time. the same time. So you could think of it as two genes at the same time. So remember in the monohybrid cross he was looking at one characteristic. He was looking at flower color. That's just one characteristic. Here, you know, look at this example. There's two things that are different. He was studying the peas, but he's not just looking at pea color, right? Yellow pea versus green pea, right? That's one characteristic would be pea color. But what do you think, take a look closely, what do you think the other characteristic or gene was that he was studying here? Can you beat Wicket? That's right, Wicket. It's the shape, right? The shape. So Mendel was looking at color, which is one gene, one characteristic, and shape, which is a completely different gene, a completely different characteristic. So the color characteristic had yellow and green, the shape characteristic had round versus wrinkly or shriveled. Okay, does that make sense? And by the way, yes, Mendel started with true breeders. So you see in the parent generation, this is the parent generation. This is not just any round yellow pea plant. This is a true breeder round and... true breeder yellow. He made sure, he made sure that ahead of time that this is a true breeder for both round and yellow. Okay, that means that he also started with a true breeder for both green and wrinkly on this side. Does that make sense? That was a given. You have to start with true breeders in these hybrid cross experiments, otherwise it's not done as a true hybrid experiment. So knowing that, He went ahead and he fertilized an egg with a sperm. You know, there's a cross-fertilization, cross-fertilization between these two plants. And what did he see? Take a look. What do you see? 100% of the offspring were yellow. And not only that, 100% were round. So what does that tell you about the yellow and the round traits? Are those the dominant traits? That's right, Wicket. Those are the dominant traits. So it appears that round is the dominant trait for shape and yellow is the dominant trait for color. Doesn't that mean that this parent on the left, that parent is dominant-dominant for everything? So what do you think is the genotype, the genotype of that parent? Let's talk about it. Is it round? So capital R means round, and how many big R's should this parent have? How many big R's? Two big R's, because it's a true breeder, right? It has two of the same allele. Two big R's means round, round, which would make this parent what? Little r, little r, wrinkly, wrinkly, right? Wrinkly, wrinkly. Now, what about yellow? Would this parent be yellow? Yes. How many big Y's for yellow? Two, which would make this parent? Too little y's for, you know, the recessive trait green. Does that make sense? This parent is big R, big R for round round, big Y, big Y for yellow yellow. This one's little r, little r, little y, little y. Now, do you think we could quickly do a Punnett square analysis? Do we know the genotypes of the parents? Yes. Could we figure out the gamete types? Why not? Let's do it. So let's say this is the sperm producer, okay? All right. Now, what could be in the sperm? Could both R's go into the sperm? Nope. Could both Y's go into the same sperm? Nope. That would make the sperm. diploid. Does that make sense? But couldn't the sperm receive shape information as well as color information? Why not? Why wouldn't a sperm contain shape information and color information? Those are two completely different genes. Those are two completely different characteristics. Does that make sense? So the same sperm can have an R and it can have a Y, right? It just can't have two R's and two Y's. You know, does that make sense? What about the egg from this parent? What could be in the egg from this parent? Can it have a little r? Sure. Can it have a little y? Sure. Okay, now let me ask you this. Did we solve it all? Are there any other sperm that parent could make besides a big R big Y sperm? I don't think so. And is there any other eggs this parent could make besides a little r little y egg? That, no. So this is another super simple square, isn't it? Like if you think about punnett squares, this is one of the easier ones. We're going to set up our square, right? We're going to set up our square. It's going to be a one by one square, and the sperm types are on this side, the egg types are on that side. What does the sperm say? Big R, big Y. What does the egg say? Little r, little y. Solve it. So if the sperm fertilizes the egg, what do we get in our offspring? This is the zygote. This is the offspring zygote. It's going to be a big R with a little r. By the way, students get confused. When you have like letters, you always put the like letters together and then the capital letter first. I hope that makes sense. So big R, little r, and then what else? A big Y and a little y. Does that make sense? So what that means is this offspring is going to be round and yellow because it has at least one big R and one big Y. It's going to be round and yellow and it's going to be a non-true breeder for both. It's a big R little r, big Y little y, big R little r, big Y little y. So although 100% of these offspring are round and yellow, they are all non-true breeders. That non-true breeders means you don't have two of the same. allele. By the way, a quick aside for you, some more very important terminology. When you have two of the same allele, Mendel would have called this true breeder. Does that make sense? So big R, big R would make you a true breeder round. However, in current terminology, we call this homozygous. Homozygous. So This would be big R, big R would be homozygous dominant and little r, little r would be homozygous recessive. Does that make sense? And Mendel would have called this, if you have a big R, little r, Mendel would have called that a non-true breeder, right? However, currently we call this a heterozygote, a heterozygous, right? So just be aware of that, okay? Now, can we figure out the next step? Let's do the next step. Can we figure out what's going to happen next? But before I get into what happens next, let me just back up a little bit, because I always like to make sure students know what's going on and why these things are happening. Do you want to know why Mendel wanted to do this experiment to begin with? So there was this concept that traits are linked. different characteristics are linked. Okay, so that would mean that genes are linked. Okay, so for example, do you see how that parent is round and yellow while this parent is wrinkly and green? So there was, there was confusion at the time. Remember, this was early on in the 1800s. There was some confusion whether different characteristics are linked or if they're independent. right? If different characteristics are linked, that means round always has to accompany yellow. Does that make sense? Because they're married together, they're linked. This would mean that different characteristics are linked. If different characteristics were linked, also green would be married to wrinkly. So basically you would never see Another combo besides yellow and round and green and wrinkly, right? However, if the traits are independent if they're not married together Well, then you would figure that out too because you might see in the f2 generation some new combos Like what would be a new combo? How about round and green? That'd be a new combo, right? How about wrinkly and yellow that would be a new combo, right? That would suggest that the traits are independent, the characteristics are independent, right? So which one's going to win out? That's why he did this experiment. So let's figure out what's happening next. Let's figure it out. Ready? Let's do that. All right, again, we know the genotype of the F1 generation. It's big r, little r, big y, little y. What do we have to do next in order to get our F2 generation? What do we need to do? That's right as always, Wicket. We need to self-fertilize this guy. This guy needs to self-fertilize itself, right? So if that's true, isn't that the same as a big R, little r, big Y, little y, cross with a big R, little r, big Y, little y? Because you're both parents. If you're both parents, you're both the same genotype, right? So can we figure that out? Do we know the genotypes of the parents? Yes. Can we figure out the gamete types? right, the types of sperm and the types of eggs? Sure, let's figure it out, right? Let's say this is the sperm producer. What could be in the sperm? Could the sperm get the big R and the big Y? Yeah, why not? So there could be a sperm with a big R and a big Y. In fact, let me write it over here and start our table right away. Let's start the table. So like I said, there's a sperm with a big R and a big Y. Is that possible? A sperm with a big R and a big Y? Sure. What about a sperm with a big R and a little Y? Is that also possible? Could we have a sperm with a big R and a little Y? Sure, big R little Y. What other combos are possible? Think about it. How about a sperm with a little R and a big Y? Is that possible? Sure. Look at how many different sperm types we can make. This is going to be a complicated square, isn't it? A little r with a big Y. And what else? How about a little r with a little y? Is that possible too? That's possible too. So, wow, four different sperm. Four different sperm. Little r, little y. Oh gosh. And if that's the type of sperm that this parent can produce, isn't that also the types of eggs? You know, we don't need to go through it again, but isn't that the same exact type of eggs this parent can produce too? So if that's true, if we're gonna make the eggs, there's four types of eggs. Those are the same. So there's a big R big Y egg. There's a big R little y egg. There's a little R big Y egg and a little R little y egg. Oh my gosh. So this is a 4 by 4 square. So it's 16 squares, isn't that? Amazing here, it's kind of big, big Punnett square, okay. 16 squares, all right. So we've set up the square, so again we've figured out the genotypes of the parents, we figured out the gamete types, we've set up the square, can we solve the square? Sure, why not, let's do it. So here we have a big R big Y sperm, fertilizes a big R big Y egg. What do we get? This offspring will be big R, big R, so it's going to be round, and big Y, big Y, so it's going to be yellow. So it's not a, it's going to be a true breeder for both. So this is going to be a, this is going to be a round and yellow P, right, round yellow offspring. What about this sperm fertilizing that egg? You know, is that possible? Sure, why not? So big R, big R. You see how we're solving this? Big Y, little y. Gosh, okay. And we could just keep going. I'm not going to do all of them, but let me show you. Let's do this last one here. Look at this. A little r and a little y sperm fertilize a little r, little y egg down here. What would this one be? Way over here. What would that one be? That's right. Little r, little r. So it would be wrinkly. And little y, little y. It'll be green. So that should give me a wrinkly green result, a wrinkly green result. But that would be, you know, the recessive, recessive, you know. So let's keep going. What about, let's do this one here. Let's do this one here. A little r, little r, so wrinkly, big Y, little y. So that would be a wrinkly yellow. But remember, that's only if traits are not linked. If the traits are linked, we shouldn't see the round yellow appear, but if the traits are independent, then we will see a round, I'm sorry, a wrinkly yellow. We'll see a wrinkly yellow here if the traits are independent, right? But that's a new combo we didn't see before. So let me show you this table already solved. Let me show you this table already filled out just for you, but you saw how we filled it in and you could solve it for yourself if you want to. But let me show it to you already filled in and let's make some assessments. How about that? All right, check it out. Here it is already filled in for you. Remember I said the very first square is going to be big R big R big Y big Y there You go big R, big R, big Y, big Y. And then I said the square at the bottom right would be little R, little R, big Y. Little R, little R, little Y, little Y, wrinkly green. And that's it. So what this is in front of you is the table already filled out for you. It's already filled out for you. And so what do we see? And by the way, yes, this is what Mendel saw. Mendel saw new combos. Okay, look at this. He saw round greens. Was round and green available? Would that be possible if shape and color were married together? If these traits were linked, would round and green be possible? No. How about wrinkly yellow? That's a new combo too, right? If wrinkly was truly married to green, would you see the wrinkly yellow offspring? No. So do you know what Mendel determined? Just by seeing this, Mendel immediately knew that traits assort independently. And so what did Mendel show? The principle of independent assortment. Isn't that cool? That different characteristics, different genes, assort independently of one another and are passed on independently of one another. And do you remember where we've seen that term before independent assortment? We've seen it in meiosis. Do you remember during Metaphase 1 of meiosis? This was termed independent assortment. And isn't that neat that Mendel, without even understanding meiosis, actually predicted something that happens in meiosis because guess what? Meiosis, independent assortment during meiosis, is responsible for these results. of independent assortment that Mendel saw. Why? Because those tetrads lined up independently, which separated different traits from one another, different characteristics from one another. So isn't that cool how Mendel, without knowing about genes or chromosomes, was able to determine pretty much what happens during meiosis, that independent assortment occurs, and that's why different traits assort independently of one another. Isn't that neat? So anyway, Let's delve a little bit further into this. So first of all, we saw different combos like wrinkly yellow and round and green. That's great because that showed that the traits are not linked. They're independent. That's great. But he saw something else. Let's talk about the phenotypic ratio of the dihybrid cross. Remember, phenotypic means what I see. Let me ask you this. How many of these are round? Round, that means dominant. for shape and yellow. That means dominant for color. How many are round and yellow? That means dominant for both genes, for both characteristics. Let's count them. One, two, three, four, five, six, seven, eight, nine. Nine are dominant dominant. Does that make sense? Dominant for shape, dominant for color. Now how many are dominant for shape? that means round, but recessive for color. How many are round but green? Round but green. Let's look. Three. Three are dominant recessive. Dominant for the first trait, shape. Recessive for the second trait, color. Does that make sense? So what about recessive dominance? This is going to be wrinkly and yellow. How many are wrinkly and yellow? Recessive for the first trait, dominant for the second trait? Three. And lastly, lastly, how many are recessive for both traits? How many are recessive, recessive, recessive? So wrinkly green, just this lone one over here. So if I were to ask you, what is the phenotypic ratio of Mendel's dihybrid cross, what would you say? 9, 3, 3, 1. And do you see why now? Isn't that neat? Now, by the way, do you think I'm going to ask you what the genotypic ratio of a dihybrid cross is? Probably not. Okay. You know why? Because that would have you figure out, like, you would have to sit there and figure out, okay, how many big R, big R, big Y, big Ys are there? How many big R, big R, big Y, little Ys are there? How many big R, little R, big Y, big Ys are there? How many big r little r, big y little y's are there? And it just goes on and on and on. It would be some really weird combo. It would be like 1 to 2 to 2 to 2 to 2. I mean, it would be some really wacky stuff. So this is why I'm trying to tell you, usually on exams and stuff, they're not going to ask you what's the genotypic ratio of a dihybrid cross because the answer would be really, really long, you know. So does that make sense? Usually for the Dihybrid Cross we focus on the phenotypic ratio which is 9, 3, 3, 1. And again, what did the Dihybrid Cross teach humanity? It taught us that different traits are, they assort independently. So just because I have brown eyes and brown hair, it doesn't mean that brown eyes are married to brown hair and I could never ever see brown hair with blue eyes, right? These traits are... These characteristics are independent of one another and meiosis, believe it or not, explains all that. Isn't that cool? Dr. Dr. D,

Dr. D, Dr. D, explain stuff. All right, welcome back. Let's go ahead and get started with chapter 14 covering Mendel and heredity.

Mendel is known as the father of modern genetics and that's because Mendel discovered the basic principles of heredity by breeding garden peas. These pea plants are called Pysum sativa and they're amazing. They helped us to crack the genetic code and to understand how heredity works. Isn't that neat? before Mendel and his experiments, along with a handful of other researchers like T.A.

Knight, we really didn't understand heredity very well. This was in the 1800s. And in the early to mid 1800s, humanity had no clue how heredity worked.

We didn't know how traits are passed on from generation to generation. Like you have brown eyes and, you know, your child might have brown eyes, but we didn't. really know how this heredity worked.

In fact, at the time, blending hypothesis was a big idea in how heredity works. And that is that your children are a blend of you. And Mendel really helped to dispel and disprove this concept of blending. And so let's delve into his findings and how he used this really... humble pea plant in order to crack some of humanity's greatest mysteries regarding heredity.

Mendel was a very interesting character in the history of science. Mendel actually went to the University of Vienna and studied science under some brilliant minds, including Christian Doppler, who taught him how to do a proper scientific study, as well as famous botanists of the time as well. So he learned about botany.

He learned about the pea plant. He learned about how to conduct a proper scientific study. So he was always interested in science, but he went on to become a monk.

And then later on, in the very abbey gardens of the monastery he worked at, he actually set up genetic crosses using these pea plants, different variety of pea plants. What a brilliant and interesting person combining both. His passion for, you know, religion as well as his passion for science, setting up these genetic studies, the results of which were groundbreaking and earned him the name the father of modern genetics. He was instrumental at discovering how heredity worked. At the time, keep in mind, this was the mid to late 1800s.

People didn't even know about the chromosomes or genes or how important genes are. And yet he was able to deduce using the humble pea plant how heredity works, you know, these concepts of passing on your traits to your offspring and how those traits manifest in your... progeny.

Isn't that neat? And let's see what his contributions were. Let's discover and learn about his contributions and how he helped to understand how heredity works. One reason why the pea plant was a great choice for Mendel and others to use to understand heredity was that so many different varieties were available.

And these varieties had different... what? mendel called characters characters are heritable features that vary among the individuals so for example flower color if we're talking you know the pea plant was a flowering plant and flower color would be an example of a character but you know that how there's different versions of that character like you could have purple flower color you could have a variation which is a white flower color so the variants were called Does that make sense?

The character was Flower Color, and to Mendel, the traits were the variations. for a character such as purple or white. I hope that makes sense.

Again, character would be flower color. Purple or white would be traits. Just like, for example, Mendel would say eye color is a character. Blue eyes are a trait. Brown eyes are a trait.

So brown and blue are traits of the character eye color. I hope that makes sense. So let's remember this terminology as we move forward.

Speaking of variety. No kidding, look at this. There were so many different variety of these pea plants.

You could look at flower color, purple versus white. So again, flower color would be the characteristic, purple and white being the traits. You could look at seed color characteristic, yellow seeds versus green seeds being the traits, seed shape being the characteristic, round versus wrinkly.

Those are the traits. Pod color. You could study pod color.

There were green variants and yellow variants or traits. Pod shape, inflated pods, constricted pods, flower position, stem length. Look how much variation existed in these pisum, sativum, pea plants.

So much variety. Great when you're studying heredity because heredity is the study of how these traits are passed on to the offspring. That's great if you have all this variety to study. Other advantages of using the pea plant?

Short generation time, you know these things grow quite quickly. Large numbers of offspring because each pea can be planted. You can think of each pea as an embryo and each pea can be planted and gives rise to a whole new plant so you can have large number of offspring.

And this is really interesting. Mating could be controlled. Plants could be allowed to either self-pollinate, that means fertilize themselves, or could be cross-pollinated.

This means that one plant could be used to fertilize a separate plant. Let me explain this a little bit more detail at the board. The pea plant was a brilliant model organism to use because unlike... animals which require a male and a female to breed to have offspring. The pea plant could do one better and let me explain why.

If you have a pea plant over here, here I have a pea plant with a purple flower. This pea plant you could cross it with another pea plant right as you would expect. You could take pollen which pollen contains the sperm. You could take pollen from this pea plant and then inseminate this other pea plant with that pollen and you could have offspring, right? However, the brilliance in using these pea plants in heredity studies is that not only can these flowering plants produce pollen which contains the sperm, but they also produce the eggs as well.

So a single flower produces the sperm and the egg, which means effectively you don't need to have another plant. You could what's called self-fertilize these plants. Does that make sense? Just allow the sperm to travel a short distance and fertilize the eggs.

inside of the flower. So the flower again, it contains the sperm and the egg. If you can make sperm and egg you could fertilize yourself and this is known as self-fertilization.

And then you could study the offspring. You could study the offspring of the self-fertilization. So this is why the plant was amazing.

You could either cross-fertilize with a separate plant or you could self- fertilize that plant with itself that's just something you can't do with animals such as mice Here's how it all works. This is a flower such as those found on Pysum sativa, the pea plant, the flowering pea plant. This is a purple flower obviously.

Now the flower can contain anthers, which are the male portion of the flower, and the anther can form pollen at the tip of these structures, these filamentous structures. And remember the pollen contains the sperm. Now the flower also possesses the carpal, the female portion of the flower, which contains these eggs. All the pollen has to do is travel from the tip of the anther to stick to the stigma, travel down the style, and fertilize all these little eggs.

And once those little eggs are fertilized, those fertilized eggs will be the ones that will be the most important. eggs become the peas, the peas in the pea pod. Isn't that neat? So you can think of a pea as a embryo, a small fertilized embryo, right?

And that embryo, when you plant that pea, it's like planting an embryo, which becomes the new plant. It's a seed. It becomes the new plant. So that pea could give rise to another purple flower plant.

Does that make sense? So What you do if you want to cross-fertilize a plant, you simply take a paintbrush and you pick up some pollen from one flower and you deposit on another flower, right? So that's how you cross-fertilize.

Now to prevent self-fertilization, what Mendel would do is cut off the anther from a immature flower, right? Wait for... you know, the flower to form.

But before the flower is mature, cut the anther off, making it impossible for self-fertilization to occur, and then to inseminate that flower, you know, that denuded flower with pollen from another plant. Does that make sense? So this would be what you need to do to cross-fertilize.

If you wanted to self-fertilize, well then don't cut the anther off. Leave the... leave the anther on and if you leave the anther on you could allow for self-fertilization to occur i hope this makes sense so again these are amazing plants to use amazing model organisms because you can either self-fertilize and see the offspring or cross-fertilize to see offspring all right let me share with you something so important this was a total mystery at the beginning of this quest to understand heredity for for Mendel and other researchers such as T.A. Knight, this is what started it all. What they tried was to self-fertilize these plants and they noticed something very bizarre when they did.

So if they had a purple flower plant and they self-fertilized this purple flower plant, what would you expect to see? What would you expect the offspring to look like if you took a purple flower plant? and you self-fertilized it, you used its own pollen to fertilize its own eggs using just this one plant, what would you expect all the offspring to look like?

That's right, Wicket. You would expect them to all be purple, right? And that's what he would see for this plant.

Like this plant, he would self-fertilize it and 100% of all the offspring would be purple flower. purple flower Plants and that makes perfect sense right your purple flower you self fertilized yourself You should have purple flower offspring and that's what he saw that that makes perfect sense But here's where it got weird you ready now he would mosey over to this purple flower plant You know same pie some sad of them same species and he would do the same thing. He would say okay Purple flower plant.

I'm going to use its pollen to fertilize its own eggs. Great. And this time something weird would happen.

This time he had mostly purple, but guess what? Some white. Some white flower plants would emerge as well.

So in this case some or most of his offspring were purple. And some were white. That's really weird, right? I mean, this plant, if you self-fertilize it, it will always give you purple flower offspring.

This plant right next to it, you know, he would self-fertilize it and it would give you purple and sometimes white offspring. Is that not bizarre? That was the big mystery, right? That was, that was what really got them thinking. And this is, this is something very important because there's genius behind this.

This was the genius. You guys ready? What Mendel and others like Knight would call this plant is a true breeder. And we need to know that this he called a true breeder. Okay?

And the reason he called it a true breeder is because when you self-fertilize it with itself, it always looks like itself. Its offspring always look like itself. It doesn't matter how many offspring you make. This one, on the other hand, is that a true breeder?

No, because sometimes white flower plants would emerge. It's like, where did that come from? Right.

So he called these ones non true breeders, non TBs for short, non true breeders. OK, so this is very important to understand because of this next step. Oh, and before I forget one more very strange thing. He also had white flower variants of these. pisum sadova plants with white flowers.

And you know what's strange? When he would self-fertilize these white flowers, they would always have white flower offspring, but he could never find a white flower plant that was not a true breeder. Isn't that strange?

So the mystery thickens, right? The plot thickens. With the purple flower plant, he noticed some purple flower plants were true breeders, meaning they would always look like themselves when you self-fertilize. And some of his purple flower plants were non-true breeders.

So of course this led to a cross where Mendel wanted to know just what happens when you cross a purple flower plant with a white flower plant. But this was the genius part. You ready for the genius part? He didn't take just any purple flower plant.

What do you think he took? A true breeder, TB for short, true breeder purple flower plant. And he crossed it with a true breeder white flower plant.

Does that make sense? By the way, like I said before, every white flower plant he found was a true breeder. So that wasn't hard to find a true breeder white flower plant. It was the purple flower plants that were sometimes true breeders and other times not true breeders. Right.

So this was the genius of it all. They made sure Mendel made sure to start with a true breeder purple flower plant. and cross it with a true breeder white flower plant.

And that is what made this study work. If he had picked a non-true breeder purple, then that would have really not made much sense. Okay, so does that make sense?

So at this point, you can do what's called a hybrid hybridization or a hybrid cross. And so Mendel did a mono hybrid cross with this. When he took sperm. or pollen from one flower and used it to fertilize the eggs on the next flower.

So let's see how that turned out. Alright, so to start out, Mendel did his famous mono-hybrid cross experiment. What does the mono-hybrid cross mean? Mono refers to a single characteristic that he was studying. In this case, the characteristic was flower color.

And there are two traits for flower color, purple and white. Remember, I previously mentioned that Purple flower plants could either be true breeders or non true breeders. Here was the genius of his experiment.

He always started with true breeders, right? So true breeder purple crossed with a true breeder white. And that's what a hybridization refers to.

Hybridization occurs between two true breeders. So when you cross a true breeder purple with a true breeder white, this is an example of a hybridization. And it's called a monohybrid again because it's a single trait, or I should say a single characteristic that he was studying.

Now, what happened? During this cross-fertilization, pollen from one plant is used to fertilize the other flower, the other plant. And then we could study the offspring.

Let's see what he saw. So check it out. 100% of the offspring of this hybrid cross were purple flower plants.

He could not find a single white flower plant offspring. And that is amazing. So that was a very curious observation.

And By the way, it already negated the entire concept of the blending hypothesis. Remember the whole blending hypothesis? People thought that your offspring were a blend of you and your spouse. And so here we see there's no blending going on. These flowers were not purpley white.

They were just as purple as this purple parent right here. There wasn't a hint of white flower color. in this purple offspring. By the way, at this point I want to show you some terminology.

The original parents are called the parent generation, that makes sense, or sometimes it's the P generation or even the P1 generation. These are the parents. The offspring are called the filial generation 1, or the F1 generation for short. Filial refers to your son, you know, or at least in Latin. So the F1 generation are the offspring of the parent generation.

So when I say I'm referring to the F1 generation, I'm talking about the offspring of the original parents, the true breeder parents. So this is the offspring of two true breeders, and it looks like the white information is missing. So at this point.

You know, Mendel and others had to scratch their head, you know. Where did the white information go? Think about that.

Where did the white information go? Is it missing? Did it never get any white information?

Did it get destroyed? Did the white information get destroyed? What happened to this white trait?

Is it in there somewhere? Is the white trait in this organism? but it's being masked somehow. You know, all of these were equally possible, you know, outcomes at the time, until they did the experiment, right?

So what they did, what Mendel did, this was the beauty in using plants as the model organism here, was that he took this plant and he self-fertilized these plants, self-fertilized. Again, take the pollen from that flower, Used it to fertilize that very flower on that very plant, self-fertilizing these to see the outcome. And let's take a look at the outcome. All right, take a look at this. When this plant was self-fertilized, it resulted in offspring that most of the time were purple.

But guess what? White made a comeback. And Mendel knew enough, you know, with his studies at University of Vienna and all of his training as a scientist, he knew enough to do this experiment hundreds of times. And so he was able to establish that not only does white color come back, it comes back.

back one quarter of the time. So what Mendel saw was that of the offspring of this self fertilization, three are purple for every one white and he called this the three to one ratio, the three to one phenotypic ratio. Let me define that term real quick, phenotype. The phenotype is what we see, it's the trait we see, we see purple and white. So what would you say in this F2 generation?

That's right, the grandchildren are called the F2 generation. And what do we see? We see three purples for every one white. And so Mendel saw a three to one phenotypic ratio, three purples for every one white in the grandchildren of the parent generation. When you self-fertilize the hybrid, This is why it's, this is, this is what we're talking about.

This is the hybrid, the hybrid self-fertilized, and in its offspring you see a three to one purple to white ratio. Now white came back, white came back, so what does that tell you? That tells you that the white information was still in this purple flower somewhere, in this plant somewhere. It was just being masked.

It wasn't missing. The white information wasn't missing. It didn't get destroyed, it was in there somewhere.

It was just being masked. So the purple information was somehow masking the white information. And somehow in the F2 generation, it presented itself again.

So this led him to a very important finding. Let's talk about that. What this showed Mendel was that each individual has information from both parents.

Okay, so this individual, not only does this individual have purple color information from the purple parent, so it has the purple trait from the purple parent, but this individual also inherited white trait information from the other parent. So What's really interesting was that although the characteristic was flower color, each individual possesses two traits for every characteristic. So if the characteristic is flower color, I can possess purple information and white information both.

I can possess both. So... So each individual possesses two traits for every one characteristic.

And not only do you possess them, one can be dominant to the other. In this case, purple trait information was masking or dominant to the white information, which is called recessive. I hope you know where we're going with this.

By the way, just to give you some foreshadowing, nowadays we don't use the terms characteristic anymore. Instead of characteristic, we say gene, right? And we don't use the term trait so much. We use the term allele. Okay, so if a characteristic was, you know, a general characteristic like flower color, now we call that flower gene, the flower color gene.

Okay, and... Different versions of that gene are called alleles. So purple would be an allele, white would be an allele.

Of what? Of the flower color gene. Mendel referred to these as characteristics instead of genes, and as traits instead of alleles, but now we know the current terminology.

And that's really interesting, right? So just like you have two parents, you know, and, you know, The parents have, you know, you inherited eye color from each of your parents. Well, you could have inherited blue eye color allele from your mom and brown eye color allele from your dad, right?

So you have two alleles for that gene. You inherited blue eye color allele from your mom. You inherited brown eye color allele from your dad. One.

is dominant to the other, therefore you may have brown eyes, right? But that doesn't mean you don't have the blue allele information inside, it's just being masked. So just to give you some foreshadowing of what you're seeing here, this flower... has the purple allele from that parent and the white allele from that parent. Then when this plant self fertilized, its offspring have different outcomes.

This one right here shows the white allele outcome, the white flower, and the rest of the three show the dominant color. So they're called the purple dominance. And three dominants appear for every one recessive.

So we say, again, the monohybrid cross results in a three-to-one phenotypic ratio of dominance to recessives every time. And so, again, the big takeaway here is that each individual inherits one allele from, you know, each parent, one allele. from each parent, so each individual has two alleles for every one gene, two traits for every one characteristic, and you pass on one of those two to your offspring. Does that make sense? For example, this plant here has a purple, let's call it capital P for purple, capital P allele refers to the purple allele.

And we'll do the lowercase p to refer to the white allele, right? So the purple allele is from this purple parent, and the lower p allele, the white allele, is from this parent. Does that make sense?

So each individual has two alleles for every characteristic, for every gene. If that's true, let me ask you this. If that's true...

Shouldn't this parent also have two alleles for the flower color characteristic or two alleles for that gene? That's true. But what would both alleles say?

If this is a true breeder, what do you think both of these alleles would say? Can you beat Wicket? Obviously, exactly right, Wicket.

This parent does have two alleles, but guess what? They both say purple, and that's why it's a true breeder. Isn't that neat?

The fact... that this has a small p, a white allele, makes it a non-true breeder. Now we know what non-true breeders are, you guys. Remember, this was baffling early on to Mendel and others, right? Like, what in the world are these non-true breeders?

How come some of my plants, when you self-fertilize them, they only give offspring that look like them? And how come sometimes when I self-fertilize them, I get a different outcome? Well, it's because, is it a Non-true breeder, you know, does it have two different alleles?

Or is it a true breeder? Does it have two of the same allele? Isn't that interesting? You're getting it now? Hopefully we're making the concepts come together.

And then what should this parent be? That's right, two little peas, right? Now let me ask you this.

Was Mendel ever able to find a white flower plant that was a non-true breeder? That's right, Wicket. He wasn't. And do you see now why?

Do you see why he couldn't find a non-true breeder white? Well, it's because you can't have anything other than little p, little p, and be white color, because if you have a big P, you're going to be purple. So the only way to show the white trait is to have two white alleles, have two white traits, right? Does that make sense? So the true breeder whites...

have to be little p, little p. And so there's no such thing as a non-true breeder white flower because that's the recessive gene. If there was another allele, you'd show that other trait.

You wouldn't show the white trait. Isn't that neat? So again, true breeder purple, big P, big P. True breeder white, little p, little p.

Look what this, look what happened. Mendel said we have two alleles for every. characteristics, so we, or gene, right? See, two alleles, two alleles, two alleles, but do you pass on both of them to your offspring?

Did this parent pass on both of these peas? Did this parent pass on both of their peas or just one? Just one, right? So that's another important thing that Mendel said, was that you possess two alleles, but you pass on one or the other.

to your offspring. Does that make sense? So this parent passed on one big P allele, one purple allele. This parent passed on one little p, one white allele, and so this individual, this hybrid inherited one allele from one parent and the other allele from the other parent, and so this led to what Mendel called the principle of segregation of alleles, and this was the famous outcome of this monohybrid experiment. So if we ask you what was the main takeaway?

What was the main point to doing this monohybrid cross? What did it teach us as humans? It taught us the principle of segregation of alleles, that each individual has two alleles for every one gene, but you pass on one or the other at random to your offspring.

Your offspring only get one or the other. Does that make sense? And those come together randomly in the fertilized offspring.

Isn't that neat? Again, remember the principle of segregation of alleles suggests that each individual has two alleles, and those alleles segregate into the progeny. So if that's true, let's see if we can figure this out.

This parent, again, was big P, big P, two purple alleles. This parent was little p, little p. Remember, the lower case refers to the recessive allele.

So in this case, little p is the white allele. Now, this parent, let's say this parent made the sperm. Let me ask you this. What did we learn about meiosis in the previous chapter?

Are sperm cells haploid or diploid? You got it, Wicket. Sperm cells are haploid. Gametes are haploid. Do you know the reason?

Do you know the reason why each individual has two alleles? Well, it's because one allele is from one homologue, one chromosome from one parent. So let's say this big P is on chromosome one from its mom, from the egg, and the other big P is on its chromosome one from its dad, you know, from the sperm. So the two Ps are on homologous chromosomes is what I'm trying to say. Do sperm cells get both homologues?

Do they get both chromosomes from both parents, or do they just get one? Do you remember that from meiosis? In meiosis, the sperm are haploid.

This means that only one of these two, it could be either one, right? But only one of these two alleles gets packaged into any one sperm because the sperm are haploid. If the sperm got both P's, then the sperm would be diploid. Does that make sense?

Hopefully that makes better sense. So what could be in the egg then? The egg gets either this little pea or this little pea, but either way the egg gets a little pea allele, the sperm gets the big pea allele, and when the sperm fertilizes the egg, what is the result of that sperm fertilizing this egg?

That's right. The zygote has to be big pea, little pea, and that's what this flower is. This plant is a hybrid.

It is a non-true breeder. It's got a big P and a little p. And now this is a good time to introduce a term that you need to know.

It's called genotype. Okay, genotype refers to the alleles that you possess. It's your allele set. So when I say what's the genotype of this plant, You could tell me it's big P, little p. It has a purple allele and a white allele.

That's why it's a non-true breeder. But if I were to ask you, what's the phenotype of this plant? Well, then you have to use your eyes, right?

It's purple. Does that make sense? Are you following along? So genotype refers to what alleles I possess, like big P, little p, little p, little p, big P, big P.

These are genotypes. phenotypes are what I actually see. I see purple. I see white.

I see purple. You see what I'm saying? All right.

So what did we do next? Do you recall? The next thing we did, didn't we self-fertilize this F1 offspring? The F1 offspring was self-fertilized to give the F2 offspring, right? So what is it?

If we're both parents, if we're the mom and the dad, if we're both parents, isn't that like... Taking a big P, little p, and crossing it with yourself, a big P, little p, does that make sense, right? Wouldn't that be the same as a cross between two non-true breeders, I guess, if you're both parents? So if that's true...

You know, what kind of sperm could you make, right? What kind of, let's say the sperm, right? What could be in the sperm, okay? Let's see if you can beat wicket.

Yeah, sure, why not wicket, yeah. A big P, so a purple allele could be in a sperm. Could a little P be in the same sperm?

No, because that would make the sperm diploid. So that means the other sperm can get what? What could be in the other sperm?

That's right. The little p could be packaged into one sperm. The big p could be packaged into the other sperm.

This is what we're talking about, principle of segregation of alleles. You have both alleles, but they segregate into the gametes, and then they recombine into the progeny upon fertilization. So here, what about eggs? Let's say this is the mom, the eggs, you know, this is the mom and the dad in one.

But what could be in the eggs? That's right, a big p egg. Or could you make a little p egg?

Sure. Does that make sense? So you can make both gamete types, right?

Now, let me ask you this. Let me ask you this. Could this sperm with a big P fertilize this egg with a little P, with a big P as well?

Could that big P sperm fertilize this big P egg? Sure. And what would the result be? What would the genotype of the outcome be? Big P, big...

P, right. Big P, big P. Great. Now let me ask you this. Would it be equally possible for this big P sperm to fertilize the little P egg?

Sure, why not, right? So that would be big P, little P. Sure, okay. And would it be possible for this little P sperm to fertilize the big P egg? Sure.

And by the way, always put the capital letter first if you can when you're pairing these up. Okay. And so, and what's the final combo? What's the final fertilization event? What if the little p sperm fertilized the little p egg?

Is that possible? Why not? Why not? So what would that give you? Little p, little p.

And let me ask you this. Would that series of random events explain the whole thing? Mendel's observations in the F2 generation?

Would it explain your three to one phenotypic ratio? It would, look! You would get three purple color plants because remember you just need one capital P to show the purple color and you would end up with only one out of these four events, one white color offspring.

Does that make sense? Only one little p little p. So we exactly showed it.

So guess what? Doesn't that mean, let me ask you this, let me ask you this, doesn't that mean that the first plant should be a true breeder? Like what if I self-fertilized this guy?

What should I see in the offspring? Exactly right, Wicket. This plant should be a true breeder. If I self-fertilize this plant, it should only give me purple offspring. But what about these two plants?

What if I self-fertilize these two plants? Ah, there you go. It's like self-fertilizing this guy. Isn't that the same? So I should see a 3 to 1 ratio of purples and whites if I were to self-fertilize these, right?

And then what about this one? What should that give me? If I self-fertilize this white flower plant, I should only see white flower offspring. So do you want to see what Mendel saw? Let's check it out.

All right, look at this you guys. This is exactly what Mendel did. He went that extra step and did exactly what we said. He self-fertilized this and saw the offspring.

What did the offspring show? It was a true breeder. That's right.

This was a big pea, big pea. Does that make sense? And what about these two?

What did he show? That's right. The white flower came back and it came back in a three to one ratio.

So again, these are big pea, little peas. Okay, and what did he show with the white flower when he self-fertilized it? That's right.

It is a true breeder, little p, little p. And so let's wrap this up. Let me tell you something important, right? What was the phenotypic ratio?

And by the way, anytime you're asked on a test, what's the phenotypic ratio or the genotypic ratio? You need to refer to the F2 generation. They're talking about the F2 generation. Okay, now what was the phenotypic ratio, the phenotypic ratio of the monohybrid cross? What was it?

Can you beat Wicket? Perfect. Three dominants to every one recessive.

You see my three purples for every one white? Three dominants to every one recessive. My phenotypic ratio for the monohybrid cross is always three to one.

Now, let me ask you this. What's my genotypic ratio? So what does that refer to?

That means the types of genotypes, right? The ratios of genotypes in the F2 generation. So let me write this down.

The genotypic ratio. So let me ask you this. Here's how you figure out the genotypic ratio.

You go to the F2 generation and you ask yourself, what do I have? Look at this. What's the genotype of this offspring?

Isn't it big P, big P? Okay, that means dominant, dominant. How many dominant dominants do we have?

How many dominant dominants do we have? True breeder dominants. That's right, I have one, right?

Now let me ask you this. How many non-true breeders do I have? That means how many genotypes do I have where it's dominant recessive, right?

Big P, little p, right? How many of those do we have? Two, right?

Two. So I have one big P, big P, two big P, little p's, and now how many little p, little p's do I have? I only have one. So what is my genotypic ratio for the monohybrid cross? It is 1 to 2 to 1. One big P, big P for every two big P, little p's for every one little p, little p.

It's a 1 to 2 to 1 genotypic ratio, which gives me my 3 to 1 phenotypic ratio. And that is exactly what you need to know for Mendel's famous monohybrid cross. All right, so in the dihybrid cross, Mendel was studying two characteristics at the same time.

the same time. So you could think of it as two genes at the same time. So remember in the monohybrid cross he was looking at one characteristic.

He was looking at flower color. That's just one characteristic. Here, you know, look at this example.

There's two things that are different. He was studying the peas, but he's not just looking at pea color, right? Yellow pea versus green pea, right?

That's one characteristic would be pea color. But what do you think, take a look closely, what do you think the other characteristic or gene was that he was studying here? Can you beat Wicket? That's right, Wicket.

It's the shape, right? The shape. So Mendel was looking at color, which is one gene, one characteristic, and shape, which is a completely different gene, a completely different characteristic. So the color characteristic had yellow and green, the shape characteristic had round versus wrinkly or shriveled.

Okay, does that make sense? And by the way, yes, Mendel started with true breeders. So you see in the parent generation, this is the parent generation.

This is not just any round yellow pea plant. This is a true breeder round and... true breeder yellow.

He made sure, he made sure that ahead of time that this is a true breeder for both round and yellow. Okay, that means that he also started with a true breeder for both green and wrinkly on this side. Does that make sense?

That was a given. You have to start with true breeders in these hybrid cross experiments, otherwise it's not done as a true hybrid experiment. So knowing that, He went ahead and he fertilized an egg with a sperm. You know, there's a cross-fertilization, cross-fertilization between these two plants. And what did he see?

Take a look. What do you see? 100% of the offspring were yellow.

And not only that, 100% were round. So what does that tell you about the yellow and the round traits? Are those the dominant traits?

That's right, Wicket. Those are the dominant traits. So it appears that round is the dominant trait for shape and yellow is the dominant trait for color.

Doesn't that mean that this parent on the left, that parent is dominant-dominant for everything? So what do you think is the genotype, the genotype of that parent? Let's talk about it.

Is it round? So capital R means round, and how many big R's should this parent have? How many big R's?

Two big R's, because it's a true breeder, right? It has two of the same allele. Two big R's means round, round, which would make this parent what? Little r, little r, wrinkly, wrinkly, right?

Wrinkly, wrinkly. Now, what about yellow? Would this parent be yellow? Yes. How many big Y's for yellow?

Two, which would make this parent? Too little y's for, you know, the recessive trait green. Does that make sense?

This parent is big R, big R for round round, big Y, big Y for yellow yellow. This one's little r, little r, little y, little y. Now, do you think we could quickly do a Punnett square analysis? Do we know the genotypes of the parents? Yes.

Could we figure out the gamete types? Why not? Let's do it.

So let's say this is the sperm producer, okay? All right. Now, what could be in the sperm?

Could both R's go into the sperm? Nope. Could both Y's go into the same sperm? Nope. That would make the sperm.

diploid. Does that make sense? But couldn't the sperm receive shape information as well as color information?

Why not? Why wouldn't a sperm contain shape information and color information? Those are two completely different genes.

Those are two completely different characteristics. Does that make sense? So the same sperm can have an R and it can have a Y, right? It just can't have two R's and two Y's. You know, does that make sense?

What about the egg from this parent? What could be in the egg from this parent? Can it have a little r?

Sure. Can it have a little y? Sure. Okay, now let me ask you this. Did we solve it all?

Are there any other sperm that parent could make besides a big R big Y sperm? I don't think so. And is there any other eggs this parent could make besides a little r little y egg? That, no.

So this is another super simple square, isn't it? Like if you think about punnett squares, this is one of the easier ones. We're going to set up our square, right?

We're going to set up our square. It's going to be a one by one square, and the sperm types are on this side, the egg types are on that side. What does the sperm say? Big R, big Y. What does the egg say?

Little r, little y. Solve it. So if the sperm fertilizes the egg, what do we get in our offspring?

This is the zygote. This is the offspring zygote. It's going to be a big R with a little r.

By the way, students get confused. When you have like letters, you always put the like letters together and then the capital letter first. I hope that makes sense.

So big R, little r, and then what else? A big Y and a little y. Does that make sense? So what that means is this offspring is going to be round and yellow because it has at least one big R and one big Y.

It's going to be round and yellow and it's going to be a non-true breeder for both. It's a big R little r, big Y little y, big R little r, big Y little y. So although 100% of these offspring are round and yellow, they are all non-true breeders.

That non-true breeders means you don't have two of the same. allele. By the way, a quick aside for you, some more very important terminology.

When you have two of the same allele, Mendel would have called this true breeder. Does that make sense? So big R, big R would make you a true breeder round. However, in current terminology, we call this homozygous.

Homozygous. So This would be big R, big R would be homozygous dominant and little r, little r would be homozygous recessive. Does that make sense?

And Mendel would have called this, if you have a big R, little r, Mendel would have called that a non-true breeder, right? However, currently we call this a heterozygote, a heterozygous, right? So just be aware of that, okay? Now, can we figure out the next step?

Let's do the next step. Can we figure out what's going to happen next? But before I get into what happens next, let me just back up a little bit, because I always like to make sure students know what's going on and why these things are happening. Do you want to know why Mendel wanted to do this experiment to begin with?

So there was this concept that traits are linked. different characteristics are linked. Okay, so that would mean that genes are linked. Okay, so for example, do you see how that parent is round and yellow while this parent is wrinkly and green? So there was, there was confusion at the time.

Remember, this was early on in the 1800s. There was some confusion whether different characteristics are linked or if they're independent. right?

If different characteristics are linked, that means round always has to accompany yellow. Does that make sense? Because they're married together, they're linked. This would mean that different characteristics are linked.

If different characteristics were linked, also green would be married to wrinkly. So basically you would never see Another combo besides yellow and round and green and wrinkly, right? However, if the traits are independent if they're not married together Well, then you would figure that out too because you might see in the f2 generation some new combos Like what would be a new combo?

How about round and green? That'd be a new combo, right? How about wrinkly and yellow that would be a new combo, right? That would suggest that the traits are independent, the characteristics are independent, right? So which one's going to win out?

That's why he did this experiment. So let's figure out what's happening next. Let's figure it out.

Ready? Let's do that. All right, again, we know the genotype of the F1 generation. It's big r, little r, big y, little y. What do we have to do next in order to get our F2 generation?

What do we need to do? That's right as always, Wicket. We need to self-fertilize this guy.

This guy needs to self-fertilize itself, right? So if that's true, isn't that the same as a big R, little r, big Y, little y, cross with a big R, little r, big Y, little y? Because you're both parents. If you're both parents, you're both the same genotype, right?

So can we figure that out? Do we know the genotypes of the parents? Yes.

Can we figure out the gamete types? right, the types of sperm and the types of eggs? Sure, let's figure it out, right?

Let's say this is the sperm producer. What could be in the sperm? Could the sperm get the big R and the big Y? Yeah, why not?

So there could be a sperm with a big R and a big Y. In fact, let me write it over here and start our table right away. Let's start the table.

So like I said, there's a sperm with a big R and a big Y. Is that possible? A sperm with a big R and a big Y? Sure. What about a sperm with a big R and a little Y?

Is that also possible? Could we have a sperm with a big R and a little Y? Sure, big R little Y.

What other combos are possible? Think about it. How about a sperm with a little R and a big Y? Is that possible?

Sure. Look at how many different sperm types we can make. This is going to be a complicated square, isn't it? A little r with a big Y.

And what else? How about a little r with a little y? Is that possible too?

That's possible too. So, wow, four different sperm. Four different sperm.

Little r, little y. Oh gosh. And if that's the type of sperm that this parent can produce, isn't that also the types of eggs? You know, we don't need to go through it again, but isn't that the same exact type of eggs this parent can produce too?

So if that's true, if we're gonna make the eggs, there's four types of eggs. Those are the same. So there's a big R big Y egg. There's a big R little y egg.

There's a little R big Y egg and a little R little y egg. Oh my gosh. So this is a 4 by 4 square. So it's 16 squares, isn't that?

Amazing here, it's kind of big, big Punnett square, okay. 16 squares, all right. So we've set up the square, so again we've figured out the genotypes of the parents, we figured out the gamete types, we've set up the square, can we solve the square? Sure, why not, let's do it.

So here we have a big R big Y sperm, fertilizes a big R big Y egg. What do we get? This offspring will be big R, big R, so it's going to be round, and big Y, big Y, so it's going to be yellow. So it's not a, it's going to be a true breeder for both. So this is going to be a, this is going to be a round and yellow P, right, round yellow offspring.

What about this sperm fertilizing that egg? You know, is that possible? Sure, why not? So big R, big R.

You see how we're solving this? Big Y, little y. Gosh, okay. And we could just keep going. I'm not going to do all of them, but let me show you.

Let's do this last one here. Look at this. A little r and a little y sperm fertilize a little r, little y egg down here. What would this one be? Way over here.

What would that one be? That's right. Little r, little r. So it would be wrinkly. And little y, little y.

It'll be green. So that should give me a wrinkly green result, a wrinkly green result. But that would be, you know, the recessive, recessive, you know. So let's keep going.

What about, let's do this one here. Let's do this one here. A little r, little r, so wrinkly, big Y, little y. So that would be a wrinkly yellow. But remember, that's only if traits are not linked.

If the traits are linked, we shouldn't see the round yellow appear, but if the traits are independent, then we will see a round, I'm sorry, a wrinkly yellow. We'll see a wrinkly yellow here if the traits are independent, right? But that's a new combo we didn't see before.

So let me show you this table already solved. Let me show you this table already filled out just for you, but you saw how we filled it in and you could solve it for yourself if you want to. But let me show it to you already filled in and let's make some assessments. How about that?

All right, check it out. Here it is already filled in for you. Remember I said the very first square is going to be big R big R big Y big Y there You go big R, big R, big Y, big Y.

And then I said the square at the bottom right would be little R, little R, big Y. Little R, little R, little Y, little Y, wrinkly green. And that's it.

So what this is in front of you is the table already filled out for you. It's already filled out for you. And so what do we see?

And by the way, yes, this is what Mendel saw. Mendel saw new combos. Okay, look at this. He saw round greens. Was round and green available?

Would that be possible if shape and color were married together? If these traits were linked, would round and green be possible? No.

How about wrinkly yellow? That's a new combo too, right? If wrinkly was truly married to green, would you see the wrinkly yellow offspring?

No. So do you know what Mendel determined? Just by seeing this, Mendel immediately knew that traits assort independently. And so what did Mendel show?

The principle of independent assortment. Isn't that cool? That different characteristics, different genes, assort independently of one another and are passed on independently of one another. And do you remember where we've seen that term before independent assortment? We've seen it in meiosis.

Do you remember during Metaphase 1 of meiosis? This was termed independent assortment. And isn't that neat that Mendel, without even understanding meiosis, actually predicted something that happens in meiosis because guess what?

Meiosis, independent assortment during meiosis, is responsible for these results. of independent assortment that Mendel saw. Why?

Because those tetrads lined up independently, which separated different traits from one another, different characteristics from one another. So isn't that cool how Mendel, without knowing about genes or chromosomes, was able to determine pretty much what happens during meiosis, that independent assortment occurs, and that's why different traits assort independently of one another. Isn't that neat? So anyway, Let's delve a little bit further into this.

So first of all, we saw different combos like wrinkly yellow and round and green. That's great because that showed that the traits are not linked. They're independent.

That's great. But he saw something else. Let's talk about the phenotypic ratio of the dihybrid cross.

Remember, phenotypic means what I see. Let me ask you this. How many of these are round?

Round, that means dominant. for shape and yellow. That means dominant for color. How many are round and yellow? That means dominant for both genes, for both characteristics.

Let's count them. One, two, three, four, five, six, seven, eight, nine. Nine are dominant dominant. Does that make sense? Dominant for shape, dominant for color.

Now how many are dominant for shape? that means round, but recessive for color. How many are round but green?

Round but green. Let's look. Three. Three are dominant recessive. Dominant for the first trait, shape.

Recessive for the second trait, color. Does that make sense? So what about recessive dominance? This is going to be wrinkly and yellow.

How many are wrinkly and yellow? Recessive for the first trait, dominant for the second trait? Three. And lastly, lastly, how many are recessive for both traits?

How many are recessive, recessive, recessive? So wrinkly green, just this lone one over here. So if I were to ask you, what is the phenotypic ratio of Mendel's dihybrid cross, what would you say?

9, 3, 3, 1. And do you see why now? Isn't that neat? Now, by the way, do you think I'm going to ask you what the genotypic ratio of a dihybrid cross is?

Probably not. Okay. You know why?

Because that would have you figure out, like, you would have to sit there and figure out, okay, how many big R, big R, big Y, big Ys are there? How many big R, big R, big Y, little Ys are there? How many big R, little R, big Y, big Ys are there?

How many big r little r, big y little y's are there? And it just goes on and on and on. It would be some really weird combo.

It would be like 1 to 2 to 2 to 2 to 2. I mean, it would be some really wacky stuff. So this is why I'm trying to tell you, usually on exams and stuff, they're not going to ask you what's the genotypic ratio of a dihybrid cross because the answer would be really, really long, you know. So does that make sense? Usually for the Dihybrid Cross we focus on the phenotypic ratio which is 9, 3, 3, 1. And again, what did the Dihybrid Cross teach humanity?

It taught us that different traits are, they assort independently. So just because I have brown eyes and brown hair, it doesn't mean that brown eyes are married to brown hair and I could never ever see brown hair with blue eyes, right? These traits are... These characteristics are independent of one another and meiosis, believe it or not, explains all that. Isn't that cool?

Dr. Dr. D,

Transcript for:Mendel's Impact on Genetics and Heredity

Transcript for:
Mendel's Impact on Genetics and Heredity