A normal couple has their first child, who is haemophilic. Work out a cross to show how it is possible. State the possibility of the normal and the haemophilic children, along with their sexes, that can be born to them. [3 marks]

Haemophilia is a X-linked recessive disease. It is possible to have a haemophilic child (boy) when a normal couple has their first child. The cross is given below: [1 mark for cross]
Let,
XY = genotype of the normal father
XX${}^h$= genotype of the normal mother
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)

[1/2 mark for each]
The possibility of normal girl = 2/2 = 1
The possibility of haemphilic girl = 0
The possibility of normal boy = 1/2
The possibility of haemophilic boy = 1/2