homozygous

If an individual's two copies of a gene are a matching pair — the same allele inherited from both parents — that individual is homozygous for the gene. Think of two identical keys for the same lock: whichever one is used, the result is the same.

Because both alleles are identical, a homozygote always passes that same allele to every gamete, so its offspring are predictable for this gene. We distinguish homozygous dominant (two dominant alleles, often written AA) from homozygous recessive (two recessive alleles, aa); the recessive case is the only way a recessive trait can appear.

Being homozygous is defined one gene at a time. A given individual can be homozygous at some loci and heterozygous at others across its genome. Whether the matching alleles produce a noticeable trait depends entirely on which gene and which alleles are involved.