An allele, or allelomorph, is a gene (see GENES) which may exist in one or more forms, only one of which can occur in a given chromosome (see CHROMOSOMES). Two alleles of a given gene are at the same relative positions on a pair of homologous (similarly structured) chromosomes. If the two alleles are identical, the subject is homozygous for the gene – namely, the genes will exert a unanimous inﬂuence on a particular characteristic. If the alleles are diﬀerent, with one having a dominant and the other a recessive inﬂuence, the subject is heterozygous.
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