The most common form of hemophilia is a defect in blood clotting factor VIII, which is caused by a mutant form of a gene on the
X chromosome. Boys who inherit that mutation from their mother suffer from uncontrolled bleeding. Girls carrying one copy of this mutation have near normal blood clotting. Uncommonly, a girl is born with hemophilia even though both parents have normal phenotypes. Which of the statements might explain hemophilia in a girl born to parents with normal blood clotting phenotypes? A. A non-disjunction event in sperm production resulted in her receiving X and Y chromosomes from her father and an X from her mother; she is XXY.
B. Non-disjunction during sperm formation resulted in her receiving no sex chromosome from her father and an X chromosome from her mother; she is XO.
C. Non-disjunction during egg formation resulted in her receiving two X chromosomes from her mother and an X chromosome from her father; she is XXX.
D. Non-disjunction during egg formation resulted in her receiving no X chromosome from her mother but an X from her father; she is XO.
The correct answer is - B. Non-disjunction during sperm formation resulted in her receiving no sex chromosome from her father and an X chromosome from her mother; she is XO.
Explanation:
In this case, the mother was a carrier of hemophilia and she transfers that mutant allele to her daughter and as the daughter did not get any sex chromosome from her father.
Therefore, mutant X expresses itself and she is hemophilic.
This X did not express in case, of her mother due to fact that it is a recessive allele and only expresses when both genes is the same state.
