Hemophilia A, a condition in which blood does not clot properl...

Question

Hemophilia A, a condition in which blood does not clot properly, is a recessive trait located on the X chromosome (Xʰ). Suppose that a woman who is heterozygous for this trait (XXʰ) has children with a normal male (XY). What is the probability that the couple will have daughters with hemophilia? What is the probability that the couple will have sons with hemophilia?

Accepted Answer

Welcome everyone. Here's our next question. A couple has four Children, two boys and two girls. One of the boys is affected by hemophilia and one of the girls is a carrier of the mutant allele for hemophilia, an excellent recessive trait which of the following is most likely true. A the mother and father are unaffected by hemophilia. B, the mother is affected and the father is unaffected by hemophilia. See, the mother is a carrier and the father is unaffected by hemophilia or d the mother and father are both carriers of the mutant allele. So let's think briefly and recall what an X linked recessive trait means. It would be a trait carried on the X chromosome, which is as it says, recessive. And that means that females need to inherit two copies and males only one to express the trait since males only have one X chromosome, even though it's recessive, all they need is one to express the trait. So let's think about that in the context of our family. So we have two boys, two girls, our boy is affected. So that means the boy must have XH. So the gene for hemophilia on the X chromosome. And then Y and then the girl that is a carrier will say boy, this is affected. And then the girl that's a carrier, then has one XH and one X, she's not effective because she has only one copy of the gene, but she is a carrier of the gene and can pass it on to her Children. So we'll say carrier and then we have another boy who is unaffected and therefore is just Xy and another girl who is not a carrier who then would be just XX. So we'd say not affected. So in order to have these offspring with these genotypes, let's look at our different answer. Choices and determine if they would be likely to lead to this. So choice A says, mother and father are unaffected by hemophilia. So that would imply that the mother is not a carrier. But if the father is unaffected, that would mean he has no gene for hemophilia. And since the mother is not a carrier, that would mean neither parent can pass on the gene. This is a genetic disorder inherited from parents. So choice a definitely wrong. The other thing we could look at is choice D right away said the mother and father are both carriers of the mutant allele, but a father would not be a carrier. If a father has the gene, he expresses the trait he's affected by hemophilia. No choice. D wouldn't be the correct description either. Now, let's look at choice B, the mother is affected and the father is unaffected by hemophilia. Well, an affected mother would have to have two copies of the gene. So she would have XHXH. So therefore she could have no unaffected sons. Sons inherit their single X chromosome from their mother. If all their mother has is an X chromosome with the hemophilia gene, all of her sons will have hemophilia. So since we know that only one of the boys is affected, choice B cannot be our answer. So all we have left here is choice C, the mother is a carrier and the father is unaffected by hemophilia. And this will be our answer. The father is unaffected because if the father had hemophilia, then all of his daughters would be carriers. You wouldn't have a daughter who isn't since they would all just be, have to, all the daughters would inherit his ex with the hemophilia gene. And the mother is a carrier, meaning she passes the gene on half the time. So if we look at a quick punnet square, so we'll put the father across the top. Our father is unaffected. So he has just the plain X and Y. The mother is a carrier. So she has one XH and one X. And as we fill in our square, we see on the top left XHX, there's a carrier of female XX. There's a female with no hemophilia gene. Top right xhy. There's our affected boy and then Xy, there's our unaffected boy. So we see that the offspring would reflect this most likely result with one affected boy, one girl who's a carrier and then a boy and a girl who are unaffected. So again, if we have this, these phenotypes among the offspring, the most likely true scenario is choice C the mother is a carrier and the father unaffected by hemophilia. See you in the next video.

Key Concepts

X-Linked Inheritance

Genotype and Phenotype

Punnett Square

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