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Question

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]

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Solution

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
XXh = genotype of the normal mother
Cross: XY x XXh
Gamtes X Y
X XX (normal girl) XY (normal boy)
Xh XXh(normal but carrier girl) XhY (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


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