PEDIGREE QUESTION (BIOLOGY) PLEASE HELP!!!! (if you could include a pedigree diagram, that'd be awesome!)?

Question

5.	The great-great maternal grandmother of a boy was a carrier for color-blindness, an X-linked disorder. His great uncle on his mother’s side was colorblind but this great uncle’s father was unaffected. The boy’s mother has 2 brothers (1 colorblind, 1 unaffected) and 1 sister (unaffected). The boy’s grandmother on his mother’s side had 1 brother who was colorblind and 3 sisters. Two of these sisters were unaffected and one was a carrier. The boy’s great grandmother on his mother’s side had 4 sisters. The boy has one unaffected sister and he is colorblind. What is the probability of the boy’s sons being colorblind if he marries a non-carrier?

Script · Accepted Answer

All you need to know from the above information is that's it's X-linked and that the boy marries a non-carrier. The other information is extraneous to the question.

The boy has ZERO chance of passing on colorblindness to his sons. His sons will receive his Y chromosome, which does not carry the colorblindness gene. His daughters on the other hand will all be carriers, as they will receive his X chromosome.

Punnett square:

Xc = colorblind X chromosome
Boy: XcY
Wife: XX

.....Xc.......Y
X...XXc.....XY 
X...XXc.....XY

Anonymous · Answer

I think it's sex-linked recessive. The first generation has a mother who is a carrier and a father who is neither affected nor a carrier. They give birth to a female who is a carrier and an affected male. Going down further, you see that all of the males in the family are affected, and none of them are merely carriers. This suggests sex linkage, because in a male, if they inherit a recessive trait on the X chromosome, they will express it because they have no chance of having it "covered up" by the dominant chromosome. A female, on the other hand, has two X chromosomes, so if one has the recessive allele and the other one has the dominant allele, then the dominant allele will mask the recessive one and the female will not be affected. Thus, females can be carriers of sex-linked traits. If it was autosomal recessive, then neither of the offspring would be affected because the father in generation 1 is not even a carrier of the trait. So, if we're calling the dominant allele 'R' and the recessive allele 'r', the mother would be Rr and the father would be RR. There is no way to get the genotype 'rr' (the 'affected' genotype) by mating these two individuals, so that rules out autosomal recessive. Meanwhile, you can't have carriers in autosomal dominant, because anyone with the dominant allele expresses the trait. The other two inheritance patterns are more complex and are difficult to represent in a pedigree, so I don't think either of them is the correct answer. Maybe for a visual aid, you can show a different pedigree of sex-linked inheritance and contrasting it to one that shows autosomal inheritance. Or, maybe you can show a picture of a patient with a sex-linked recessive genetic disorder (e.g. Fragile X syndrome).

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PEDIGREE QUESTION (BIOLOGY) PLEASE HELP!!!! (if you could include a pedigree diagram, that'd be awesome!)?

2 Answers