Question

Why is it that the father never passes on the gene for haemophilia to his sons? Explain.

Answer 1

The reason the father never passes on the gene for haemophilia to his sons is due to the nature of its inheritance pattern, which is sex-linked recessive and specifically linked to the X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). A male with haemophilia has the defective gene on his single X chromosome. When he passes on his chromosomes to his offspring, he gives his Y chromosome to his sons and X chromosome to his daughters.

Since sons inherit the Y chromosome from their father and the X chromosome from their mother, they never receive their father's X chromosome carrying the haemophilia gene. Therefore, a father cannot pass haemophilia to his sons. However, all daughters of a haemophilic father receive his affected X chromosome and become carriers (if the mother’s X is normal) or affected if the mother also contributes an affected X chromosome. This explains why haemophilia appears in the sons of carrier mothers and not directly from haemophilic fathers to sons.

In summary, the father passes his Y chromosome (not the affected X) to his sons, preventing transmission of haemophilia to them, but passes the affected X chromosome to his daughters, who can be carriers or affected depending on the mother's genotype.