For the following crosses, determine as accurately as possible...

Question

For the following crosses, determine as accurately as possible the genotypes of each parent, the parent in whom nondisjunction occurs, and whether nondisjunction takes place in the first or second meiotic division. Both color blindness and hemophilia, a blood-clotting disorder, are X-linked recessive traits. In each case, assume the parents have normal karyotypes.
A man and a woman who each has the wild-type phenotype have a son with Klinefelter syndrome (XXY) who has hemophilia.

A man and a woman who each has the wild-type phenotype have a son with Klinefelter syndrome (XXY) who has hemophilia.

Accepted Answer

Hi, everyone. Let's look at our next problem. It sets a man with color blindness and a woman with hemophilia have a daughter with Turner syndrome X O who is unaffected by both traits. What is the probability that the couple's next child will be a daughter with Turner syndrome? Once more choice? A 25%. B 12.5% C 75% or D cannot be determined. Well, this question is actually simpler than it looks at first glance. First of all, when we look at our scenarios here, we see that the father has color blindness, we know he only has one X chromosome. So that gene for color blindness must be on his single X. So I'm gonna write X superscript C to symbolize an X with a gene for color blindness. And then Y as his genotype. And then it says that the mother has hemophilia, hemophilia is a recessive ex linked trait. So the mother with her two X chromosomes must carry the gene for hemophilia on both since it's a recessive disorder. So I'm gonna put some superscript H on both her XES to show that she has the gene for hemophilia on both. Well, the next part of the problem then says that their daughter has Turner's Syndrome. So her genotype, as our problem tells us, daughter is X O. Turner syndrome is when a female has only a single X chromosome and then is missing the second sex chromosome. Well, it says she's unaffected by both traits if she has complete Turner syndrome, a complete lack of that second chromosome. This is impossible because all of the X chromosomes, she could possibly have inherited, she's only inherited one X chromosome from one of her parents, either mother or father. But all of these X chromosomes have a gene for the traits. And since she has only one X, if she inherited the color Linus gene from her father. So if the X comes from the male side, she'll be color blind because she'll inherit that gene on her single X. If the X comes from the female side, she'll have hemophilia. Since both of the mother's exes carry that gene. However, there is a possibility that her form of Turner syndrome is mosaic Turner syndrome. And in this case, the X is only partially missing or missing in only some of the cells. So some cells in her body are missing an X, some are not. And in that case, we know also that in females, the X chromosome is inactivated randomly in each cell early in development. So early in development, when there's just a few cells in each cell, one of the X is is inactivated randomly. So some every and then every cell descended from those cells has the same x inactivated. So some cells in the body will have an X from the mother and some from the father in normal development. And in this case, that's complicated by the fact that some cells will be completely missing an X chromosome. Well, in this case, since there's two different disorders, you could have a situation where some cells carry the colorblind gene, some cells carry the hemophilia gene and some have no X chromosome. So you could have a situation where they all sort of complement each other causing the daughter not to express pheno typically either of these traits. So that's obviously kind of a really complicated story for why she wouldn't be affected by either trait. However, that's not what our question is asking us. Our question is simply asking us, what is the probability that the couple's next child will be a daughter with Turner syndrome once more? Well, this doesn't require pundit squares or anything like that because Turner syndrome is rarely inherited from a parent. It's usually the result of an error in meiosis or cell division and therefore only affects the single individual. So it's actually very unlikely that their next child will have Turner syndrome, especially given that this daughter seems to have the mosaic form of Turner syndrome that most likely will only affect the one daughter as it wouldn't have been inherited from either parent. But at any rate, we have this convenient choice d cannot be determined. And that will be our answer. We cannot determine without any further information. And it's unlikely we can determine given that it was probably a spontaneous mutation arising. The odds that another child would be a daughter with Turner syndrome. See you in the next video.

Key Concepts

X-linked Inheritance

Nondisjunction

Klinefelter Syndrome

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