A woman whose father does not have hemophilia gives birth to a son with classic hemophilia. This woman is found to have only 15% of the normal amount of clotting factor VIII (the clotting factor males with hemophilia are missing) and does have abnormal blood-clotting issues. Which phenomenon or factor is most likely responsible for her abnormal clotting factor expression?
A)The X chromosome most commonly inactivated in her bone marrow is paternally derived.
B)The X chromosome most commonly inactivated in her bone marrow is maternally derived.
C)This woman is demonstrating reduced penetrance of an X-linked-recessive disorder.
D)This woman is demonstrating full penetrance of an X-linked-recessive disorder.

Question

Quizplus · Accepted Answer

It is most likely the woman's mother (grandmother of the infant with hemophilia) who is the carrier of this X-linked abnormal gene. For autosomes, the expected percentages of maternal and paternal chromosomes inherited in any cell, organ, or tissue is 50% for each. However, because women inactivate one X chromosome randomly in all cells, which becomes "fixed" early in embryonic development, genes from the X chromosome can be unequally expressed when percentages are different for maternally derived and paternally derived X chromosomes. This woman has only 15% expression of the genes that are from paternally derived X chromosomes in the tissues that make clotting factor VIII and 85% expression of the genes that are from the maternally derived X chromosome (with the abnormal and nonfunctional gene).

A Woman Whose Father Does Not Have Hemophilia Gives Birth