In X-linked inheritance, the gene of interest is on the X chromosome, not on an autosome. Because females have 2 X chromosomes, they must inherit two copies of the disease allele to express the disease phenotype. Females with only one mutated allele are carriers.

Males are more frequently affected because they only have one X chromosome and need only one allele mutated to express a disease phenotype. All males who inherit a copy of the abnormal X chromosome are affected by the disease (assuming 100 percent penetrance).