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. A genetic pedigree is shown. Irene is a carrier. Her husband is not a carrier. Her children Waldemar and Henry have hemophilia.

What is the best evidence to prove that Irene was heterozygous for hemophilia?
Alice carried the recessive allele.
Alexandra carried the recessive allele.
Frederick carried the recessive allele.
Waldemar carried the recessive allele.