heterozygous

An individual is heterozygous for a gene when its two copies are different alleles — one version inherited from each parent. Picture two different keys on the same ring: one of them might fit the lock and the other not, so what you observe depends on which version takes charge.

In simple Mendelian cases, one allele is dominant and the heterozygote shows the dominant trait while quietly carrying the recessive one. Such an individual can pass either allele to a gamete with equal chance, so two heterozygous parents can produce offspring of several genotypes — the source of the classic 3:1 phenotype ratio.

Not every heterozygote looks like the dominant homozygote. With incomplete dominance the two alleles blend into an intermediate phenotype, and with codominance both alleles are expressed at once. The word simply means 'two different alleles here'; how they interact is a separate question.