Question

what is the best evidence to prove that irene was heterozygous for hemophilia?
elizabeth is not a carrier for the trait.
tatiana carried the recessive allele.
her brother has hemophilia and her sister is a carrier.
two of her sons have hemophilia and one does not.

Explanation:

Hemophilia is an X - linked recessive disorder. For a woman (Irene) to be heterozygous (\(X^{H}X^{h}\)), she would pass the recessive allele (\(X^{h}\)) to some of her sons. Sons inherit the X - chromosome from their mother. If two sons have hemophilia (\(X^{h}Y\)) and one does not (\(X^{H}Y\)), it indicates that she has both the normal (\(X^{H}\)) and the hemophilia - causing (\(X^{h}\)) alleles.

• Option 1: Elizabeth not being a carrier gives no information about Irene's genotype.
• Option 2: Tatiana being a carrier is related to Alexandra's genotype (Alexandra is a carrier as she has an affected son Alexei) and not directly to Irene.
• Option 3: Her brother having hemophilia is due to their mother's genotype (assuming their father has a normal X - chromosome as males with hemophilia usually have limited reproductive success in the past context of such pedigrees). But this does not directly prove Irene's heterozygosity.

Answer:

Two of her sons have hemophilia and one does not.