Question

A 25-year-old woman has mild expression of hemophilia A. A genetic diagnosis reveals that she is a heterozygous carrier of a mutation in the X linked factor VIII gene. What is the most likely explanation for mild expression of the dis- ease in this individual?

A. A high proportion of the X chromosomes carrying the mutation are active in this woman
B. Her father is affected, and her mother is a heterozygous carrier
C. Nonsense mutation causing truncated protein
D. One of her X chromosomes carries the SRY gene
E. X inactivation does not affect the entire chromosome

Answer 1

The mild expression of hemophilia A in a heterozygous carrier female is likely due to the higher proportion of X chromosomes with the normal factor VIII gene being inactivated in her cells.

Step-by-step explanation:

The most likely explanation for the mild expression of hemophilia A in a 25-year-old woman who is a heterozygous carrier of the mutation is X inactivation, commonly known as lyonization. Carrier females can manifest mild forms of hemophilia because of inactivation of the X chromosome carrying the normal factor VIII gene, while the X chromosome carrying the mutation remains active. This woman would have a random inactivation of one of her X chromosomes in each cell, and if by chance, a higher proportion of her cells inactivate the X chromosome with the normal factor VIII gene, she would express mild symptoms of hemophilia.