What is the best evidence to prove that Irene was heterozygous for hemophilia?

Hemophilia is typically an X-linked recessive trait. Sons inherit their only X chromosome from their mother, so Irene must have passed a hemophilia allele to her two affected sons and a normal allele to her unaffected son. This is the best evidence that Irene was heterozygous, meaning she carried one hemophilia allele and one normal allele.

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