heterozygous

In a living organism, having two different alleles for a given trait. In homozygous organisms, by contrast, both chromosomes carry the same allele. In an outbreeding population an individual organism will generally be heterozygous for some genes but homozygous for others.

For example, in humans, alleles for both blue- and brown-pigmented eyes exist, but the ‘blue’ allele is recessive to the dominant ‘brown’ allele.

Only individuals with blue eyes are predictably homozygous for this trait; brown-eyed people can be either homozygous or heterozygous.