Answer
The boy's X chromosome without mutated alleles likely arises from X chromosome inactivation during embryonic development.
Work Step by Step
1. Gene Mutations in Hemophilia A and B:
a. Clotting factor VIII gene mutation causes hemophilia A.
b. Clotting factor IX gene mutation causes hemophilia B.
2. Heterozygous Woman Carrying Mutated Alleles:
a. A woman may carry mutations in both factor VIII and factor IX genes, making her heterozygous.
b. One X chromosome carries a mutated factor VIII allele, and the other X chromosome carries a mutated factor IX allele.
3. Sons Inherit X Chromosomes:
Sons inherit one X chromosome from their mother.
4. Sons' Risk of Hemophilia:
Normally, all sons of this woman are expected to have either hemophilia A or B, as they inherit a mutated allele from her.
5. Exception: Birth of a Son without Hemophilia:
a. In rare cases, a son born to this woman does not have hemophilia.
b. This suggests that the son's X chromosome does not carry either mutated allele.
6. X Chromosome Inactivation:
a. The explanation lies in X chromosome inactivation.
b. During embryonic development, one X chromosome in each cell of a woman is randomly inactivated to maintain gene dosage balance.
7. Inactivation of X Chromosome with Mutated Alleles:
In this woman, the X chromosome carrying the mutated factor VIII and IX alleles might be preferentially inactivated in some cells.
8. Active X Chromosome without Mutated Alleles:
The active X chromosome in the cells of the son that does not carry the mutated alleles would be the one without the mutated alleles.
9. Result: Son without Hemophilia:
As a result, the son inherits and expresses the X chromosome that does not have the mutations in factor VIII and IX genes.
