Question

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

Answer 1

Hint: Haemophilia is caused by a mutation or modification in one of the genes that codes for the clotting factor proteins necessary to form a blood clot. The clotting protein may no longer function at all or not at all as a result of this modification or mutation. These genes are found on the X chromosome.

Complete Step by Step Answer:
One of the child is normal that is one of the allele has to be normal. Suppose if their mother is homozygous for the disease that is mother is affected and the father is also affected: then both the children will be surely diseased. Now, suppose mother is heterozygous for haemophilia and father is affected then there then half of the females will be affected and half of the males will be affected.


Haemophilia is caused by a gene that is transferred from parent to kid. A mother who carries the gene is referred to as a carrier, and she has a 50% chance of bearing a son who has haemophilia as well as a 50% chance of bearing a daughter who is also a carrier.

So, option (A) is correct.

Note: Most of the genes on the X chromosome have only one copy in males, whereas females have two copies. As a result, a man who inherits an X chromosome with a mutation in the factor VIII or factor IX gene may experience haemophilia-like symptoms.