Question

Mary's father has haemophilia but her husband is also haemophlic. What is the chance of her daughter having this disease ?

• A. 0%
• B. 50%
• C. 100%
• D. 25%

Answer 1

The correct option is 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, Mary's father has haemophilia, so she would be the carrier. This is because if the father would carry affected allele, he would transmit it to his daughter.
Parent generation : X${}^h$X (Mary) x X${}^h$Y (husband)

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% carriergirl, 50% haemophlic boy, 50% normal boy.

Option B is correct.