Haemophilia is a defect in blood clotting. It is a recessive trait that has arisen from a mutation of a gene on the X-chromosome. A boy with haemophilia has parents who have normal blood clotting. If the parents have another child what is the probability it will be normal?
A) 0
B) 1/4
C) 1/2
D) 3/4
E) 3/8

Question

Quizplus · Accepted Answer

The mother must be a carrier (X^HX^h) and the father is normal (X^HY). The possible genotypes for their children are: X^HX^H (normal girl), X^HX^h (carrier girl), X^HY (normal boy), and X^hY (haemophilic boy). Thus, 3 out of 4 children will be normal.

Haemophilia Is a Defect in Blood Clotting