Example punnet square for sex-linked recessive trait | High school biology | Khan Academy

Hemophilia is an X-linked recessive trait that affects blood clotting. If someone has hemophilia, their blood has trouble clotting. If a carrier woman and a hemophiliac man have a daughter, what is the percent chance that she, the daughter, will have hemophilia? So if you're so inspired, pause this video and try to work through this on your own.

Alright, now let's work through this together. To do this, let's think about the genotypes of both the mother and the father. So let's start with the mother. Since it is an X-linked recessive trait, we're going to be dealing with the sex chromosomes.

Let's just remind ourselves that the mother, because she's female, is going to be XX; she has two X chromosomes. The father is going to have an X chromosome and a Y chromosome. But now let's think about the hemophilia. They say it’s a carrier woman. A carrier woman means that one of her two X chromosomes would have the hemophilia allele, and the other one wouldn't.

Carrier implies that she doesn't show the trait; she doesn't have hemophilia, but she's carrying one of these X-linked recessive hemophilia alleles. So we could say that one of her chromosomes, one of her X chromosomes, does not have hemophilia. I use capital H for the dominant allele, which you would say is no hemophilia. We tend to use the capital for the dominant and then the lowercase h, where this would be the X chromosome that has the hemophilia allele, and I denote it lowercase h because it is a recessive trait. So, this is hemophilia.

Now what about the father? Well, it’s X-linked, and the father only has one X chromosome. They say it’s a hemophiliac man, and so that one X chromosome is going to have the hemophilia allele. The reason why he exhibits hemophilia is that there is no other X chromosome with the dominant allele to be dominant over the hemophilia allele.

But now let's cross these two with a Punnett square. The father, to the offspring, could contribute the X chromosome that has the hemophilia allele or a Y chromosome. The mother can contribute the X chromosome that doesn't have the hemophilia allele or the X chromosome that does have the hemophilia allele.

Now I'll draw my little Punnett square here. So there you go. What are the different scenarios? Well, this top-left scenario would be I get the X chromosome from the mother that does not have the hemophilia allele and I get the X chromosome from the father that does have the hemophilia allele.

This scenario is the X chromosome from the mother with the hemophilia allele and the X chromosome from the father with the hemophilia allele. This scenario is the X chromosome from the mother without the hemophilia allele (so, no hemophilia) or with the non-hemophilia allele and the Y chromosome from the father. And this is the X chromosome from the mother that has the hemophilia and the Y chromosome from the father.

Now, what is the percent chance that she, that a daughter, would have hemophilia? So, which of these scenarios shows a daughter? Well, the daughters are going to be the XX scenarios. These are the two scenarios in which they have a daughter—these two—because there are Y chromosomes, and these would be them having a son.

So out of the two scenarios where they have a daughter, one of those scenarios shows you have two of the recessive alleles. This is a situation where you have a daughter with hemophilia right over here. If we know they have a daughter, what is the percent chance that she will be hemophiliac? Well, one out of two, so there is a 50% chance.

Now, what's useful about this Punnett square is you could answer all sorts of questions. You could say, what is the percent chance that they have a hemophiliac son? Well, one out of these four scenarios is a hemophiliac son, then you would say there's a 25% chance that they have a hemophiliac son.

Similarly, there is a 25% chance that they have a non-hemophiliac son. If someone said, what is the percent chance that they have a hemophiliac offspring? Well, that would be these two scenarios right over here. So, two out of the four scenarios are hemophiliac offspring, so that would be 50%.

If you said, given that they had a son, what is the percent chance that they don't have hemophilia? Well, you would say given that they have a son, so that would be those two scenarios; one out of those two scenarios, the son does not have hemophilia, so that would again be a 50%.

So anyway, hopefully, this was useful.