What does homozygous mean?

Homozygous refers to an organism that has two identical alleles for a particular trait. This means that both alleles can be dominant, recessive, or any combination where both are the same type, such as AA (homozygous dominant) or aa (homozygous recessive). This genetic uniformity can influence the expression of traits in an organism, impacting phenotype.

In contrast, having two different alleles indicates heterozygosity. The presence of one dominant and one recessive allele is a specific case of heterozygosity, and having a dominant trait does not necessarily specify the genetic makeup—dominance can occur in individuals that are either homozygous or heterozygous. Therefore, the definition of homozygous is specifically aligned with having two identical alleles.