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:

To determine if Irene is heterozygous for hemophilia (a sex - linked recessive disorder), we analyze the inheritance pattern. Hemophilia is X - linked recessive, so for a female to be heterozygous (a carrier), she has one normal X and one X with the recessive allele. If two of her sons have hemophilia (they inherit the recessive X from her) and one does not (he inherits the normal X from her), this shows she has both alleles (heterozygous).

• Option 1: Elizabeth not being a carrier doesn't relate to Irene's genotype.
• Option 2: Tatiana's allele status doesn't directly prove Irene's heterozygosity.
• Option 3: Her brother having hemophilia and sister being a carrier doesn't confirm her heterozygosity as strongly as her sons' phenotypes.
• Option 4: Since sons get their X from their mother, having two sons with hemophilia (recessive allele) and one without (dominant allele) shows Irene has both alleles, so she is heterozygous.

Answer:

D. Two of her sons have hemophilia and one does not.