Question

Explain the inheritance of haemophilia in the first generation with normal father and carrier mother. Indicate the genotypes and phenotypes of the progeny.

Answer 1

Haemophilia is a sex-linked recessive disorder carried on the X-chromosome. When a normal father (XY) mates with a carrier mother (X^HX^h), where X^H is the normal allele and X^h the haemophilia allele, the inheritance pattern is as follows:

Genotypes of progeny:

• Sons receive Y from father and either X^H or X^h from mother: 50% normal (X^HY) and 50% affected (X^hY).
• Daughters receive X^H from father and either X^H or X^h from mother: 50% normal (X^HX^H) and 50% carriers (X^HX^h).

Phenotypes of progeny:

• Sons: half will have haemophilia, half normal.
• Daughters: all phenotypically normal; half carriers, half normal.

Thus, haemophilia appears in male offspring but not in females, as females must inherit two defective alleles to be affected, which is very rare.