Pseudodominance

Pseudodominance is the situation in which the inheritance of a recessive trait mimics a dominant pattern.

Normally, two recessive alleles need to be inherited (one from each parent) for the recessive trait to be expressed but recessive merely means that the trait is only expressed in the absence of the dominant alleles. The pattern of inheritance in which a single recessive allele is inherited but is still expressed is known as pseudodominance. This mainly occurs with sex-linked genes (i.e., those on the sex chromosomes). The homogametic sex (females in humans) receive two of each sex chromosome and therefore need to be homozygous to show a recessive trait. The heterogametic sex (males in humans) only receives one of each sex chromosome and will therefore show recessive trait even if they have only inherited one copy of a recessive allele.