3 a colorblind man and a woman with normal vision whose...

"3 . A colorblind man and a woman with normal vision whose father was colorblind have a son and a daughter. Colorblindness, in this case, is caused by an X-linked recessive gene. Use XB for the normal allele, Xb for the mutant a) If only the male offspring is considered, what is probability that a son will be colorblind? b) What is probability that their daughter will be colorblind? c) If the daughter isn't colorblind, what is the probability that she carries the gene for colorblindness (she is a carrier)?"

Transcript

00:01 Look at a few examples of how does a recessive sex -linked trait travel across a pedigree? so the first example we're going to look at is what does it look like if the mom has this recessive sex -link trait and the dad does not.

00:23 What will their children be? well, to know that we're going to start by writing out all the genotypes that we know.

00:33 So the mom, who we know has this recessive sex link trait, that means that both of her x chromosomes have recessive alleles on them.

00:43 But the dad, who does not have this trait, his one and only x chromosome has to have the dominant allele.

00:55 So then when this couple has a son, well, the son will get y from dad and an x from mom, no matter which x they get from mom, it will be one with a recessive allele on it.

01:14 So the son will end up having this sex -linked trait.

01:22 But the female who gets one x from mom, which we already know has to be recessive, and gets an x from dad, which we see is dominant, they will be a carrier.

01:41 Now we're going to look at another example of what if the dad has this trait and the mom does not.

01:56 Well, again, you start with the genotypes that you know.

02:00 We know the dad who does have the trait has a recessive allele on his only x chromosome.

02:11 And the mom who does not have the trait has to have at least one dominant allele.

02:20 So on her other x, could be dominant, could be recessive.

02:28 We don't know entirely...