Question

The following pedigree chart shows the inheritance of haemophilia in a family.

(a) What would be the possible genotype of members 4 and 5?
(b) A blood test shows that member 14 has haemophilia gene. What would be the probability that her child will be a haemophilic boy? [3]

Answer 1

(a) Haemophilia is a X-linked recessive disease (X${}^h$). Therefore, the genotype of member 4 and 5 will be XX${}^h$ and X${}^h$Y respectively. [1]

(b) Member 14 has haemophilia gene. That means she is a carrier (XX${}^h$). [1 mark for cross]
Cross: XY x XX${}^h$

Gamtes	X	Y
X	XX (normal girl)	XY (normal boy)
X${}^h$	XX${}^h$(normal but carrier girl)	X${}^h$Y (haemophilic boy)

The probability that her child will be haemphilic boy = 1/4 = 0.25 = 25%. [1]