Explain criss-cross inheritance with reference to bleeder’s disease by suitable charts.

Answer: Haemophilia (bleeder’s disease) is caused by a recessive X-linked gene (X^h); blood fails to clot because of lack of a clotting factor. Since males have only one X, a single recessive allele makes them haemophilic. The X-linked gene passes from father to daughter and from mother to son — this is criss-cross inheritance.

Chart 1: Haemophilic man × normal woman

The father’s gene goes to all daughters (carriers); sons are normal.

Chart 2: Carrier daughter (XX^h) × normal man (XY)

Result: 50% daughters carriers, 50% sons haemophilic. Thus the trait passes from the grandfather through his carrier daughter to the grandson (criss-cross inheritance).