APA Dictionary of Psychology

n. an alternate form of a gene that occupies a given position on each of a pair of homologous chromosomes. Each person typically has two alleles of each gene: One is inherited from the mother and the other from the father. Alleles may be alike (homozygous) or different (heterozygous) and are responsible for variation in inherited characteristics, such as hair color or blood type. See also dominant allele; recessive allele. —allelic adj.