A heterozygote is an individual having two different alleles at a genetic locus; a homozygote is an individual having two copies of the same allele at a locus. The term homozygote is also sometimes applied to larger genetic entities, such as a whole chromosome: a homozygote is then an individual having two copies of the same chromosome.

(Note the noun forms 'heterozygosity' and 'homozygosity', and the adjectives 'heterozygous' and 'homozygous'.)