X-linked dominant

Here a single altered copy of a gene on the X chromosome is enough to cause the trait, and because both sexes have at least one X, both sexes can be affected. Unlike the recessive case, females are not shielded by a second X — one dominant copy already does the job.

The most distinctive feature comes from how fathers transmit their chromosomes. An affected father passes his X to all of his daughters (making every daughter affected) but his Y to all of his sons (so no son is affected). An affected mother, carrying the allele on one of her two X chromosomes, passes it to each child — son or daughter — with a 50% chance.

On a pedigree, X-linked dominant traits often show no father-to-son transmission and frequently affect more females than males. For some genes the dominant allele is so severe in males — who have no second X to soften it — that affected boys are rarely carried to term, which skews the pattern further toward affected girls.