Q.Haemophilia is more common in males because it is a

Haemophilia, red-green colour blindness (daltonism) and Duchenne's muscular dystrophy are the chapter's human examples of X-linked recessive inheritance. Haemophilia — the 'bleeder's disease' — results from a recessive X-linked allele that leaves affected individuals without a normal blood-clotting substance, so minor injuries can cause dangerous, prolonged bleeding; a carrier mother (unaffected herself) transmits it to about 50% of her sons even when the father is unaffected.

Colour blindness needs a dominant X-linked allele to build functional colour-sensing cone cells, so only XcXc females (homozygous recessive) or XcY males (hemizygous recessive) fail to distinguish red from green. Both conditions show the same criss-cross inheritance pattern: when a colour-blind man (XcY) marries a normal-visioned woman, all F1 children have normal vision but every daughter is a silent carrier (XXc) — colour blindness then reappears only in the next generation, in her sons, having effectively jumped from grandfather to grandson through an unaffected carrier daughter. When a colour-blind woman (XcXc) instead marries a normal-visioned man, the trait appears faster: every son is colour-blind in the very first generation (since a son's only X must come from his colour-blind mother), while every daughter is a carrier rather than affected.