X-LINKED DOMINANT

Here the mutant gene is dominant and is on the X chromosome. Males may be normal or affected and the female heterozygotes are more variably affected due to random inactivation of one X and severely affected when both X chromosomes carry it.

CRITERIA

1. Affected males never transmit it directly to sons, but do transmit the gene to all their daughters. Fig. 32a.
2. Affected heterozygous females transmit it to half their sons and half their daughters.
3. There are more females than males, and in some conditions it is lethal in the male. Fig. 32a.
4. Affected heterozygous females show a variable clinical picture due to random inactivation of one X.

CLINICAL EXAMPLES

1. Vitamin D resistant rickets (hypophosphataemic rickets) appears at around 6 months of age when serum phosphorus levels drop. Diminished tubular resorption of phosphorous and reduced gut resorption of calcium and phosphorous contribute to X rays showing bowing of lower limbs, slow growth with short stature and a waddling gait.
2. Incontinentia pigmenti and Rett syndrome in males are lethal and only females survive. The Xg blood group is a normal trait inherited in this manner.