APA Dictionary of Psychology

the version of a gene (see allele) whose effects are manifest in preference to another version of the same gene (the recessive allele) when both are present in the same cell. Hence, the trait determined by a dominant allele (the dominant trait) is apparent even when the allele is carried on only one of a pair of homologous chromosomes. The term autosomal dominant is used to describe this pattern; for example, Huntington’s disease is an autosomal dominant disorder.