Hi. It's Mr. Andersen and welcome to Biology Essentials video 29. This is on Mendelian Genetics and it's called Mendelian Genetics. It's named after Gregor Mendel. In biology there are two famous names. Darwin, who was famous and controversial in his own time, and Gregor Mendel who died in obscurity.
But both of them made huge advances in the field of biology. And Gregor Mendel in the area of genetics. If they could have gotten together, those two theories when they finally came together as modern synthesis, it was really powerful.
Unfortunately he dies in obscurity. But this is what he did. He was crossing pea plants.
And so he can take one pea plant and you use a paint brush to transfer pollen from one to another so you know who are the parents. He would then create offspring as a result of that. Now peas are great because they have a number of different characteristics.
And more importantly you can make a lot of them really quickly. So in a pod each of these peas is an actual new organism. So you can plant those and see how they grow. And so you figured out a lot of genetics as a result of that.
And so genetics in this podcast I'll talk about are Mendelian or simple genetics. He identified the gene and he came up with two laws. The law of segregation and independent assortment.
I'll talk about those. We'll also have some problems. So I'll have some practice problems that you can try and I'll show you how to work those out. And we'll finish with genetic disorders.
The example I'll talk about is Huntington's disease. And with genetic testing it opens up this whole idea of ethics and privacy. Now what won't I be talking about? I won't be talking about linked genes.
In other words if genes are ever on the same chromosome or on a sex chromosome or caused by multi genes, it gets really complex. And so I'll talk about those in the next podcast on advanced genetics. But know this.
That simple Mendelian genetics, the rules are simple rules of math. Rules of probability. And if you understand those and how to do the first cross that I show you, you should do well in most any of the crosses you get in genetics. So this is what Mendel did.
He crossed purple flowers with white flowers. And he got purple flowers. Now a few things that you should know.
The first cross in any genetic cross is called the P cross or the parental cross. And then the offspring of that are called the F1 or the filial one or the offspring of that cross. And so at the time of Mendel, everybody believed in this idea of blending.
That if you cross two parents, so mom and dad, their kids look a lot like them. And so it's maybe a blending of all some, they didn't know that they were genes, but something inside of them. And so when he crossed purple with white and got purple flowers, that made total sense back then.
This kind of fits in this blending. But what he did next was he crossed these purple flowers with themselves. And what he got was a 3 to 1 ratio of purple to white.
And that white returned just as crystal white as that first white was to begin with. And so what he said is that there's a character, a trait that's passed through here. But we now identify that as a gene that's carried down the gene.
And so what he did is he took a gene and he put it in a cell. And he put distinctly through each of those generations and then shows up again. Now if you know anything about genetics you'd know that in simple Mendelian genetics it looks like purple is dominant, white is recessive. But we know that because of the work of Mendel. And so this is what The second cross is the one that was puzzling.
And this is what he eventually figured out. So these offspring right here were hybrid for the trait. And so if we look at the parents, the parents would be big P, big P.
That represents one purple flower. Little p, little p represents the white flower. And so each of these would be big P, little p.
And so you can use a Punnett square. A Punnett square you put the pollen or the male here, the female right here. And each of these genes get a certain column. And so this would be one parent, big P, little p.
And so you simply write that across. So we've got big P and little p here. We've got big P and little p here. So this represents the two different parents crossing with each other. And then we simply figure out what comes.
So here's big P, big P. So we get a big P from one, a big P from the other. So that would be big P, big P. Since big P is dominant we get a purple flower. So we get On the next one I'm going to take a big P from here and a little p from here.
And so that would be still purple because this one is a dominant allele or a dominant gene. Down here we get a big P from here and a little p from here. And so that's purple. And the reason we get white flowers is that you get a little p from both of the parents.
And so the neat thing about a Punnett square, and that's what this is, is it not only allows you to quickly do the probability, but it shows you the percent we should get in those offspring. In other words three of them should be purple and one of them should be white. So we should have a 3 to 1. ratio. And if you ever get across like this in a problem, for example across like this, just if you're not sure what the offspring are going to be, just do a simple Punnett square where these two alleles or versions of the gene are going to be on the top and these two are going to be on the side. And we'll do some practice problems in just a second.
And so what are Mendel's laws? Well the two things that he figured out, the two Or the law one, Mendel's law one is called the law of segregation. And Mendel's law two is the law of independent assortment.
And so let's start with law one. And so if we ever, and I've got a coin here because the actual way you get genes are almost like a coin flip. So if you think about a coin it has heads on one side and tails on the other. When you flip a coin, what are the odds that you're going to get heads or tails? Well it's a one in two probability that you'll get heads.
Same thing with genes. And so if this is that f, One generation, and this shouldn't be B. This should be P.
So we'd say this is big P, little P. What are the odds that the offspring are going to get a big P? Well it's a 1 in 2. What are the odds that they're going to get a little p? It's 1 in 2. And so that separation of those two alleles is called segregation. And so this idea of segregation says that there's a 50% chance you're going to get either of these genes.
And so that's segregation. They separate. And it's just random chance. The next one is the law of independent assortment. The law of independent assortment says that this gene, the gene that causes for example hitchhiker's thumb, which is where your thumb actually bends back, and the gene that causes an attached ear lobe, so right here I've got a free ear lobe, those two traits don't affect each other.
In other words they sort independently. So And so we can work problems without mixing these two together. They're going to not influence one another.
Now sometimes we'll find, for example, that some things do travel together. So you'll notice that people who have red hair also have freckles. And that's because those two genes are actually found on the same chromosome.
And so they seem to travel together. And so we're not going to deal with linked genes again. We'll do that later. And so independent assortment means that traits don't affect each other. And so what I'm going to do next is I'm going to leave this for for a second.
These are six problems that I'll work through. But if you want to work these you could pause the video at this point and then you could come back and start the video again and see me work through each of these. So I'll pause. Alright.
So let me go through these. Question 1. A coin is flipped four times. It comes up heads each time. What is the probability that the next coin flipped will come up heads?
Well everything that's happened in the past can't influence anything that's going to come in the future. And so it's a one half probability that you'll get heads. In other words you could have 10 kids.
They could all be boys. What are the odds that the next one is going to be a girl? It's still a 1 in 2 probability.
Let's look at the next one. And so we've got some things up here. Round P's are going to be big R. And round P's are going to be And wrinkled peas are going to be Little r.
Yellow will be big Y and dominant is going to be, or green is going to be little Y. Generally whatever is the dominant trait, we give that the capital letter. In this case round gets the big R and yellow gets the big Y. And so question number two.
Classify the following as heterozygous or homozygous. Heterozygous means you have different genes or different alleles. Homozygous means you have the same.
And so this first one, big R, big R would be homozygous dominant. They have the same. And then we have the other one.
This is the This would be heterozygous. And we also sometimes refer to that as hybrid. The next one would be homozygous recessive. And the next one would be heterozygous yellow, homozygous for the round. So it's going to be heterozygous and then homozygous dominant on the next one.
So that tells you the alleles that you have. Let's look at number 3. What's the phenotype of the following? Well this right here is going to be the genotype. In other words big Y little y is going to be the genes that you have. What's going to be the phenotype?
Well that's physically what you look like. And so for the first one, you have the genotype. So you this one right here, even though its genotype is big Y little y or its heterozygous for that, its phenotype would be yellow.
So this is going to be yellow. This one is going to be round. This one is going to be green.
And this one here is going to be yellow round. Phenotype is physically what you look like. Let's look at the next one, number 4. What's the probability of this cross, so we have two round seeds producing wrinkled seeds. said before, if you ever get one of these, it's a simple monohybrid cross, I would always do a Punnett square. And so we'd put big R and little r on one side of my Punnett square.
Big R, little r on the other side. side of my Punnett square. So what are the odds that I am going to get wrinkled seeds?
Well there is a little r here, a little r here. And so there would be a 1 in 4. So I probability that we'd have wrinkled seeds. Because this one's going to be round.
This one's going to be round. This one's going to be round as well. And so again if you ever get a simple cross like that, do a Punnett square. Let's look at the next one.
What's the probability that this cross would produce green seeds? Well I'd do the same thing again. Big Y, little y.
And then I'd do the same thing. So I'd do the same thing. So I'd do Crossed with little y, little y. And we're looking for green seeds. Green seeds, remember, are going to be little y, little y.
And so I could get it here. I could get it here. And so that is a 2 in 4 or a 1 in 4. And so I'm going to get that. And so I'm going to and 2 probability that we're going to get green seeds from that. And so even though you might think you're super smart, do a Punnett square.
You're never going to miss the problem then. Now this is a problem that we'll sometimes get on the AP Bio test as well. If these parents are crossed together, what are the odds that you'd get that? Well to do this one you'd have to actually set up a pretty intense Punnett square. And so if you get one like this, don't do a 4 by 4 Punnett square.
Just work each of them individually. And so what do I mean by that? Well let's start on the r's.
What are the odds that if you produce this and that, you're going to You could get that. So let's do an r's first. So big R little r crossed with big R big R. What are the odds that we're going to get big R little r?
Well neither of these. But this is a big R little r. So what's the odds that we're going to get big R little r?
Well little r. This is a big R little r. And so the odds of these two parents producing these offspring is going to be a 1 in 2 probability. So I'm going to write that right over here underneath the 1 in 2 probability.
Now let's work the Y's. So let me get a, let's see. a different color.
So if we do the y's, what are the odds that these two parents are going to produce that offspring? Well let's do those together. So here are the parents.
Big Y, little y crossed with big Y, little y. So this is going to be big Y, big Y, little Y, little y, big Y, little y, big Y, little y. So what are the odds that we're going to produce again? Big Y, little y. Well it's a 2 in 4. So what are the odds that we're going to produce?
or a 1 and 2 probability. So I'm going to write 1 and 2 here. Okay. So now instead of doing this huge unwieldy 4 by 4 upon a square, what I've done is I've almost got there.
Because the odds of getting this are 1 and 2, the odds of getting this are 1 and 2. So what are the odds of getting both of those? I simply multiply those together. And it's a 1 and 4. In other words what are the odds of flipping the coin and getting heads?
1 half. What are the odds of flipping two heads in a row? It's a half times a half or a fourth. And so you can solve problems like this.
just using what's called the law of multiplication. These two things have to happen. So I hope you did well on those problems. Last thing I want to talk about is disease. And a nasty disease is called Huntington's disease.
It's named after the person who identified it in the 1800's. But essentially what you get is degeneration of the nerve fibers in this portion of your brain. And so what happens is eventually you start to get uncontrollable shakes. You can't really walk.
Eventually you die as a result of that. Now the problem with Huntington's disease is you don't know you have it until you're middle aged. So I could have Huntington's disease right now.
I'm going to die as a result of this disease but I don't know it. And so I've already had kids. I've already passed the genes on.
A famous person who had Huntington's disease is this guy. His name is Woody Guthrie. You probably know him. He wrote the...
the song, this land is your land, this land is my land. And he died as a result of having Huntington's disease. Now it's a dominant trait.
In other words, if you are this, you get Huntington's disease. If you are this, you don't. And so let's look at a pedigree.
A pedigree shows you how a disease can be passed down through organisms. And so on a pedigree a square is always going to be a male. A circle is going to be a female. And if you ever have a horizontal line between them, it means that they are the same.
So means that they had offspring. And so this is the grandparents in this case. And they had a boy.
And then the next person, the kid they had was a boy. And then they had a girl. And then they had another girl. And you can see that this girl for example had her own family.
But you can trace the disease through it. In other words since this parent right here is big, let's use a different color, big H, little h. And this one is little h, little h. This big H was actually transferred to the son. It was not transferred to this son.
It was not transferred to this daughter. But it was transferred to this daughter over here. And so the odds of passing it on are one in two.
And you can see that one in two of their kids had that. And if it's a dominant disease like this, lots of times, you can see that it's we'll see it in generation after generation after generation. But you've reproduced already by that time. And so it's almost too late. Now where does this become an ethics issue?
Well we now have a test for Huntington's disease. And so Woody Guthrie, you're, you're going Woody Guthrie had a number of kids. One of those is named Arlo Guthrie who is also a famous folk singer.
And so Arlo Guthrie may have the Huntington's gene. He has a 1 in 2 probability of getting it. We now have a test that can figure out if you have that gene. But it will influence your life in the future. And so would you want to know that you're going to get a disease that will cause a nasty death as a result of that?
There's not a lot of treatment for hunting disease or not. And would your insurance company want to know that as well? And so again the genetics behind simple Mendelian genetics are fairly simple.
But it opens up all these moral issues. And I don't have an answer for any of those questions. But it's something we're going to have to deal with in the future. And so that's genetics.
Mendelian genetics. And I hope that's helpful.