On the other hand, females must be either homozygous for the normal allele, heterozygous, or homozygous for the mutant allele, just as they are for autosomal loci. When an X-linked gene is said to express dominant inheritance, it means that a single dose of the mutant allele will affect the phenotype of the female. A recessive X-linked gene requires two doses of the mutant allele to affect the female phenotype. All daughters of an affected male and a normal female are affected. All sons of an affected male and a normal female are normal.
Males are usually more severely affected than females.
The trait may be lethal in males. In the general population, females are more likely to be affected than males, even if the disease is not lethal in males. The following Punnett Squares explain the first three hallmarks of X-linked dominant inheritance. X represents the X chromosome with the normal allele, XA represents the X chromosome with the mutant dominant allele, and Y represents the Y chromosome. Note that the affected father never passes the trait to his sons but passes it to all of his daughters, since the heterozygote is affected for dominant traits.