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(1) Of, or pertaining to an individual (or a condition in a cell or an organism) containing two copies of the same allele for a particular trait located at similar positions (loci) on paired chromosomes (see homologous chromosomes).

(2) Having two identical alleles that code for the same trait.


A ‘homozygous’ organism for a particular trait is described to possess either a pair of dominant alleles (e.g. AA) or a pair of recessive alleles (e.g. aa).

Word origin: G. ‘homo’ meaning ‘same’ + zygous, which pertains to a zygote.

Related forms: homozygosity (noun).
Related phrases: homozygous recessive, homozygous dominant.

Compare: heterozygous.
See also: allele.