Pedigrees are used to help geneticists understand how traits are inherited between generations. The Romanovs, a Russian royal family, had a history of hemophilia, a sex-linked disorder that is determined by a recessive allele on the X chromosome.

Question

Pedigrees are used to help geneticists understand how traits are inherited between generations. The Romanovs, a Russian royal family, had a history of hemophilia, a sex-linked disorder that is determined by a recessive allele on the X chromosome.

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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.

Answer 1

The best evidence to prove that Irene was heterozygous for hemophilia is that two of her sons have hemophilia and one does not. This indicates that Irene must have passed on the recessive allele for hemophilia to her two affected sons, while her unaffected son received a normal X chromosome from her. This suggests that Irene is a carrier for the trait.