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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.
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Answered by GPT-5 mini AI
Two of her sons have hemophilia and one does not.

Reason: Hemophilia is X-linked recessive. Sons inherit their single X from their mother, so a mother who is heterozygous (carrier) can pass either a normal X (unaffected son) or a mutant X (affected son). Having both affected and unaffected sons shows she must carry one mutated and one normal X.