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.
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.

Answer 1

Answer:

Waldemar carried the recessive allele.

Explanation:

The carrier is the individual that has the affected allele or mutation but does not express the trait, or might express it in different levels. Although, as the person carries the mutation, she or he might transmit the genetic mutation associated with a disease to the progeny.  In general, these diseases are inherited as recessive traits.

So, in the exposed example we know that:

• hemophilia is a sex-linked disorder
• hemophilia is determined by a recessive allele on the X chromosome.

• Irene is a carrier.
• Her husband is not a carrier.
• Her children Waldemar and Henry have hemophilia.

If Irene is a carrier, this means that she is heterozygous and that her genotype is X⁺X⁻ (Being the symbol + the dominant allele, and - the recessive one for that expresses the trait)

The fact that Irene´s husband is not a carrier means that his genotype is X⁺Y

Their boys Waldemar and Henry have hemophilia, so both their genotypes are X⁻Y

The best evidence to prove that Irene was heterozygous for hemophilia is that Alice carried the recessive allele.

• Alice is Irene´s Mother, and she is a carrier as well. Irene´s father, Louis, is not a carrier, so she could have inherited a dominant allele from her father and a recessive allele from her mother, X⁺X⁻, or she could have inherited two dominant alleles from both her parents X⁺X⁺. This is not proof enough of Irene being heterozygous.

• The fact that Alexandra, Irene´s sister, was also a carrier does not say anything about Irene´s genotype, because they could both share the same genotype or not. This is not proof of Irene being heterozygous.

• Frederick (her brother) was hemophilic.  He received a recessive allele from Alice, but this does not say anything about Irene´s genotype.  
• The fact that Waldemar (her son) was hemophilic, is the best evidence to prove that Irene was heterozygous for hemophilia. Walderman received the Y chromosome from his father and an X chromosome from his mother. The X chromosome that he received from his mother carried the recessive allele for the trait, and this is why he had hemophilia. This means that there is no best evidence for Irene´s genotype than her son´s genotype.