Inheritance of X-linked Genes

Three human conditions are named in this chapter as classic examples of X-linked gene inheritance: red-green colour blindness (daltonism), haemophilia, and Duchenne's muscular dystrophy. Haemophilia — the 'bleeder's disease', first reported by John Cotto in 1803 — is caused by a recessive X-linked allele whose carriers lack a normal blood-clotting substance (thromboplastin), so that even minor injuries can lead to prolonged, potentially fatal bleeding. Because it is X-linked recessive, women are typically unaffected carriers rather than sufferers, and a carrier mother transmits the condition to about 50% of her sons even when the father is entirely unaffected — the disease appears to 'skip' the mother and reappear in her son, the pattern the chapter calls criss-cross inheritance.

Colour blindness works the same way genetically: a dominant X-linked allele is required to build the colour-sensing cone cells properly, so only the double-recessive genotype in females (XcXc, homozygous recessive) or the single-recessive genotype in hemizygous males (XcY) fails to distinguish red from green. The chapter traces this criss-cross pattern through two contrasting marriages. When a colour-blind man (XcY) marries a normal-visioned woman (XX), every F1 child has normal vision, but every daughter is a silent carrier (XXc); only when that carrier daughter later has children of her own does colour blindness reappear — in her sons (Fig. 4.7), which is why the trait appears to leap a generation and cross from grandfather to grandson through an unaffected daughter. When, instead, a colour-blind woman (XcXc) marries a normal-visioned man (XY), the pattern is faster: every son in the very first (F1) generation is colour-blind …