Now before we get into Punnett squares and how to solve problems using them, let's talk about a few things that you need to know. You need to be familiar with alleles, different flavors of genes. And there's two types of alleles, typically for one trait. You have the dominant allele, which will be represented by a capital letter, such as capital B for brown eyes, or you have a recessive allele.
which will be the lowercase letter. In this case, it could represent blue eyes. Now, the traits can vary. It could be eye color.
It could be hair. It could be if you're tall or short. It can vary. Now, you need to be familiar with these terms as well. The first one, homozygous dominant.
So what does that mean? Well, think of the word homo. Homo means the same. So a person with a homozygous dominant trait.
would have two capital letters or two capital dominant alleles. In this case, B and B. Someone whose heterozygous has a dominant and a recessive allele. Hetero means different. Homozygous recessive, they have the same recessive allele.
In this case, two lowercase letters. So this information here is referred to as the genotype. for that particular trait because it tells you the genes that correspond to a certain trait.
Now the phenotype is associated with the physical characteristics that relate to those traits. Now the individual with the homozygous dominant trait, he's going to have brown eyes because big B is associated with brown eyes. The person with the homozygous recessive trait It's going to have blue eyes.
So this represents the phenotype of the individual, the physical characteristics that you can visibly see. Now what about the heterozygous individual, where he has both the dominant and the recessive allele? What physical characteristic will he display, or she? What will be that individual's phenotype? So what would you say?
Which one is going to win? the dominant allele or the recessive allele. Whenever you have an individual with a heterozygous trait, the dominant allele is going to win the battle.
So this person is going to have brown eyes. Let's start with this one. A homozygous wolf with blue eyes mates with a heterozygous wolf with brown eyes.
What is the probability that they will produce a wolf with blue eyes? So let's begin by drawing a monohybrid cross. So this is a Punnett square that only focuses on one characteristic, one trait, in this case, eye color.
So what we're going to do is we're going to draw one square, and then we're going to draw a vertical line. and a horizontal line. So now we have a total of four squares. Now, what is the genotype of the homozygous wolf?
So homozygous means that he has, or it has, both the same genes, either lowercase b or capital B, but they have to be the same. Now, We're dealing with blue eyes and brown eyes. Typically, blue eyes is usually the recessive trait and brown eyes is usually the dominant trait.
So here we have a homozygous wolf with blue eyes. So therefore, that wolf has the genotype lowercase b, lowercase b, which I'm going to put here. Now the second wolf is a heterozygous wolf with brown eyes.
So based on that information, what is the genotype of that wolf? So it's heterozygous, which means that it's going to have two different alleles, one capital, one lowercase, and it's still going to have brown eyes since capital B is the dominant allele. So now all we need to do is... Basically fill in the Punnett square so in this square.
We're going to write these two letters And we're going to put the capital letter first, so it's going to be capital B and then lowercase b and the same is true for this one and for this particular square it intersects these two letters so it's going to be lowercase b lowercase b and for the last one it will be the same so once we fill out the Punnett square now we can answer the question so what is the probability that they will produce a wolf with blue eyes so this represents the genotype of a wolf with blue eyes and these genotypes represent a wolf with brown eyes. So to calculate the probability you look at how many have been chosen. In this case there's two with blue eyes out of a total of four. So two out of four that's one half which is 50%. If you divide this you'll get 0.5 in your calculator and then multiply that by 100. That will give you 50%.
So that is the probability that they will produce a wolf with blue eyes. Now what about part b? Calculate the phenotype ratio and the genotype ratio. So let's make some space. Let's start with the phenotype ratio.
So we're dealing with the colors. of the eyes, the physical characteristics. And there's only two colors to deal with. Either the baby wolves have blue eyes, or they have brown eyes.
So out of, let's say, four baby wolves, two of them will have blue eyes, and two will have brown eyes, which I use the color red instead of brown for some reason. And so you could divide both numbers by two. And thus, you get the simplified ratio.
So it's a one-to-one ratio. So that's the phenotype ratio. Blue to brown, or if you want to do brown to blue, it's still going to be one-to-one. Now, let's talk about the genotype ratio.
And there's only two genotypes. So it's either big B and little b, which we see here, or... little b and little b.
So it's still 2 to 2, which can be reduced to a ratio of 1 to 1. So that's the genotype ratio for the first generation. Now, let's work on another example. Two heterozygous cats with brown eyes mate together.
What is the probability that they will produce a cat with brown eyes? So once again we're going to say capital B is associated with brown eyes and little b is going to be associated with blue eyes. So based on this information calculate the probability that the baby cat is going to have brown eyes. So let's begin by drawing a Punnett square. Now the fact that both parents are heterozygous means that their genotype is big B, little b.
So we can write that here. So in this spot, that's going to be the intersection of big B and big B. Here we're going to have big B, little b.
Same over here. And for the last one, that's going to be the intersection of two lowercase b's. So now we could focus on answering the question, what is the probability that they will produce a cat with brown eyes? So which cats will have brown eyes? So this one will be brown, and the same is true for these two.
And this particular cat will be blue. Out of four cats, three of them will have brown eyes. So the probability will be the three cats with brown eyes out of a total of four. Three divided by four is 0.75.
If you multiply that by 100, you're going to get 75%. So that is the probability that they will produce a cat with brown eyes. Now part B, what is the probability that the baby cat will be homozygous? So out of all the traits listed here, which one is homozygous? So homozygous means that they have the same letters.
In this case, it's big B and big B. That's homozygous or little b with little b. So for part B, there's going to be two cats out of a total of four with a homozygous trait or a homozygous genotype.
So that's 50%. Now let's talk about calculating the phenotype and the genotype ratio in part C. So let's start with the phenotype ratio. So once again we only have two colors, two eye colors, so that's brown and blue.
And we have three genotypes that correspond to the color brown. and only one that corresponds to the color blue. So it's going to be 3 to 1. So that's the phenotypic ratio.
Now let's calculate the genotypic ratio. So first, let's list the different genotypes that we see. So this is 1, big B and big B. These two are the same. capital B and lowercase b, and this one is the last one remaining.
So we only have one homozygous dominant genotype and we have two heterozygous genotypes and one homozygous recessive genotype. So the genotypic ratio is 1 to 1. Number three, consider a situation where incomplete dominance occurs in flowers. So capital R is associated with red, capital W is associated with white, but when you have a heterozygous genotype, RW, you get something in between, in this case the color pink.
So with that in mind, let's answer the question for part A. What is the probability that a red flower will be produced from two pink flowers? So we're only dealing with one trait, so we just need a punnett square, a 2x2 punnett square with 4 squares inside. So each pink flower will have the genotype RW. Now let's go ahead and fill the table.
So this is going to be RR. RW, RW, and WW. So what is the probability that a red flower will be produced?
So this is going to be a red flower. Here we're going to get a white flower. And let me look for a pinkish color.
So these two will be, they will represent the pink flower. So we have a total of two, well we're looking for red flowers, so we only have a total of one red flower out of four. So 1 divided by 4 is 0.25 times 100, so that gives us a probability of 25%.
So there's a 25% chance that a red flower will be produced from the two pink flowers. If we wish to calculate the probability... that a pink flower will be produced it's going to be 2 out of 4 and so that's going to be 50% and the probability of getting a white flower it's just 1 out of 4 which is 25%. Now let's move on to part b what is the probability that a pink flower will be produced from a red and pink flower? So we no longer have two pink flowers for the parents.
We have one red, one pink. So let's write the genotype. So the genotype for the red flower will be RR, and for the pink flower, it's going to be RW. So RW for pink, RR for red.
So let's go ahead and fill out the square. So this is going to be RR, and then we're going to have RW. So these two flowers... will have a red color and these two will have a pink color.
So the probability that a pink flower will be produced, there's two pink flowers out of a total of four and so that's going to give us a 50% probability. So that's it for part B. Number four. A bear with black fur and blue eyes mates with another bear that has white fur and brown eyes.
What is the probability that the baby bear will have black fur and brown eyes? So in the last three example problems, we only dealt with one particular characteristic. So we use a monohybrid cross.
But in this problem, we're dealing with two traits. the color of the fur, and the color of the eyes. So we need to use a dihybrid cross. So this problem will involve more work. So let's begin by drawing a Punnett square.
Hopefully this is big enough. And this one is going to have four columns and two columns. four rows. So 4 times 4 is 16, and so we're going to have a total of 16 squares.
Now the genotype for the first parent is going to be big F, little f, little b, little b. And the genotype for the second parent is going to be little f, little f, big B, little b. Now, what should we place here and here?
Here's what you shouldn't do. You shouldn't just write one letter in each column. This is not the right way to do it because you're dealing with two characteristics. So you need to place two letters in each column so that in each box, you're going to get four letters.
So how do we know which two letters go where? So hopefully you've taken algebra and you remember how to FOIL and that's what we're going to do. So we're going to take one letter from the first box and pair it up with a second letter from the second box that represents the second characteristic. So we're going to pair up capital F with lowercase b so that's going to give us big F little b and then we're going to pair up Big F and little b again. So if you remember the word foil, it's first.
That's for the blue line. And for the red line, outer. So that's going to be big F, little f. And then I for inner, little f, little b.
And then the last, little f, the little b again. Now let's do the same thing for the second genotype, or the genotype for the second parent. So it's going to be little f and big B, and then we have little f, little b, and then it's going to be little f, big B, and little f, little b.
So now let's fill in the Punnett square. So we have big F, little f. and then big B, little b. So this is going to take a while.
What I recommend doing is pausing the video and fill it out yourself. And then you can play the video again and see if you have what I have as well. So I'm just going to take a minute and fill everything out.
So I'm going to try to double check my work as I do this, so I don't make any mistakes. Almost done. You could fast forward this if you want to.
so now at this point we can answer part a so what is the probability that the baby bear will have black fur and brown eyes so I'm going to use can't use black because the background is black so I'm just going to use like blue for black So black fur is capital F. All we need is just one capital F. So let's identify every genotype that will show up with the black fur physical characteristic.
So all we need is just one capital F and the bear will have black fur. Now let's identify all the bears that will have brown eyes. So all we need to identify is at least one capital B.
And that's it. So now we need to identify all the bears that have both black fur and brown eyes. So this is 1, 2, 3, and 4. So we have a total of 4. Well, there's 4 that's been selected out of a total of 16 possibilities, because there's a total of 16 squares. So 4 out of 16. If you divide both numbers by 4, you can reduce the fraction to 1 over 4, which is 0.25 or 25%.
So this is the probability that the baby bear will have black fur and brown eyes. So now let's move on to Part B. What is the probability that the baby bear will have white fur and blue eyes? Go ahead and try that. So let's identify all the bears with white fur. So that means that they need to have the genotype lowercase f and lowercase f.
So this bear has white fur, this one too. Basically all the ones on the second half of the Punnett Square. They will all have white fur because they're homozygous recessive.
Now let's identify all the bears with blue eyes. So they should have the genotype little b little b since blue eyes is going to be the recessive trait or the recessive allele. So let's use the color blue.
So here's one, here's another one, that's another one, and that's it. So the bears that have both white fur and blue eyes is 1, 2, 3, 4. So once again, it's 4 out of 16, which is 1 fourth, and so the probability is 25%. Now let's move on to Part C. So what is the probability that the baby will be homozygous dominant for at least one trait? So let's think about what that means, homozygous dominant. So that means that they have to have the same alleles, either big F, big F, little F, little F, big B, big B, little B, little B.
Now, so those are the homozygous genotypes. Now we want the dominant ones, not the recessive ones, so we can eliminate these two. So we either want big F and big F or big B and big B. So let's see if we can identify any of those. And I don't see it at all.
So the probability is 0% because there's none with a homozygous dominant trait. Now, part D. What is the probability that it's going to be heterozygous for both traits? So heterozygous means that we're going to have one big letter, one little letter.
So for the first trait, it's going to be big F, little f. For the second trait, big B, little b. So here is one. Here is the second one.
That doesn't count. Not those. Here's another one.
And... Not those as well. So once again it's 4 out of a total of 16 which seems to be 25%.
25% is the number of the day. We're getting this answer a lot. So that's the probability that the baby bear will be heterozygous for both traits.
Now let's move on to part E. So we need to calculate the genotype and the phenotype ratio. So what I need to do is make some space. So let's start with the genotype ratio. So this is the first one, big F, little f, big B, little b.
And so let's identify each one with those characteristics. So it's just four. Now the next one is little f, little f, big B, little b.
And I see four of those. And then the next one is going to be big F, little f, little b, little b. And so there's also four of those.
And finally, little f, little f, with little b, little b. So 4, 4, 4, 4. If we divide everything by 4, then the genotypic ratio is 1 to 1 to 1 to 1. Now the phenotypic ratio, the characteristics, it turns out it's going to be the same. Now the genotype Big F, Little f, Big B, Little b, it corresponds to a bear with black fur and blue eyes. Actually not blue eyes, but brown eyes because of Big B. Now the second genotype, Little f, Little f, that's for white fur. Big B, Little b, that's for brown eyes.
The third genotype. Big F, little f, that's for black fur. Little b, little b, that's for blue eyes.
And the last one, little f, little f, that's for white fur. Little b, little b, blue eyes. So that's going to be four bears that have black fur and brown eyes. Four that have white fur and brown eyes.
Four have black fur and blue eyes. And the other four has white fur and... blue eyes. So it all depends on the genotypes of the parents. And so that's going to affect the genotypic and the phenotypic ratio of the first generation of baby bears.
So this will also be reduced to 1, 1, 1, 1. And that's basically it for this video. So now you know how to fill out Punnett squares and how to use them to solve problems. Thanks again for watching, and don't forget to subscribe to this channel.
So for those of you who need help in algebra, physics, chemistry, and things like that, I do have other videos on those topics. Thanks again for watching.