Principles of Inheritance and Variation Question 188

188. One child is haemophilic (sex-linked trait), while its fraternal twin brother is normal. Which one of the following informations is most appropriate?

[1999]

(a) The mother must have been heterozygous

(b) The child is a monozygotic twin

(c) The other child is a female and the father is haemophilic

(d) The haemophilic child is a male

Show Answer

Answer : a

Hints & Solution

(a) The mother must have been a haemophilic carrier $(X^{h} X)$. One of the twins would have inherited the normal $X$ chromosome and the other would have received the $X$ chromosome carrying the gene for haemophilia.