Question

If a boy's father has haemophilia and his mother has one gene for haemophilia, what is the chance that the boy will inherit the disease ?

• A. 100%
• B. 50%
• C. 0%
• D. 75%

Answer 1

Answer: (B)

50%

Haemophilia is a X-linked recessive disorder. Since, males are hemizygous for chromosome, one copy of the affected gene in each cell is sufficient to cause the disorder(X${}^h$Y). Females have two X chromosomes and hence need two copies of the affected gene to cause the disorder (X${}^h$X${}^h$). Females heterozygous (X${}^h$X) for this trait be normal but serve as a carrier of the disease. According to the question, Boy's father has haemophilia (X${}^h$Y), and mother is carrier (X${}^h$X).
Parent generation: "X${}^h$X" x "X${}^h$Y"

Gametes: (X${}^h$X) --> X${}^h$Y	X${}^h$	X
X${}^h$	X${}^h$X${}^h$ haemophlic girl	X${}^h$X carrier girl
Y	X${}^h$Y haemophlic boy	XY normal boy

F${}_1$ generation = 50% haemophlic girl, 50% carrier girl, 50% haemophlic boy, 50% normal boy.
Thus, there are 50% chances that the boy will be haemophlic.
Option B is correct.