TBR Biology Ch. 2 #12

[ajumobim]

Hemophilia A results from a deficiency of factor VIII while hemophilia B results from the deficiency of factor IX. Genes coding for both hemophilias reside on the X chromosome and are genetically transmitted as a sex-lined recessive trait. What is the probability that a woman whose father has hemophilia, and who marries a normal man, will have an affected son?
A) 1.00 B) 0.75 C) 0.50 D) 0.25

How do you know from the question that the woman's mother is normal (XX)? While solving this question, I only assumed that she did not have hemophilia, so that she could be either (XX or XXh), which would mean that the woman could be either XXh or XhXh and so the son could have a 50% chance (if the woman was XXh) or 100% chance (if the woman was XhXh)

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[NextStepTutor_1]

As it's written it seems you have a legitimate point. While there's no direct statement that the mother of this son is either homozygous or heterozygous for the hemophilia X chromosome, the only thing I can think of is that if the mother was affected the question would typically read "...a woman affected by the disease.." - by no way is this fair in the question IMO (as recognition by omission), but typically that's how it would be worded if the mother was affected (or, as you put, they'd give us the genotype, or maybe the phenotype, of this mother's mother).

Overall, your analysis is spot on, and I'm in your corner. It could have been worded better to give a cleared indication of the mother's genotype, other than omitting that information and presuming that means healthy (i.e. the mother's mother is healthy because the question doesn't mention her).

 

[Labrat07]

It's a bit tricky in question stem. All would assume she's carrier though and not affected.