In genetic terms, what does homozygous mean?

Homozygous refers to an organism that carries two identical alleles for a specific trait. This means that for a gene, both copies—one inherited from each parent—are the same, whether they are both dominant or both recessive. For example, an individual with two alleles for a trait that are both "AA" or both "aa" would be considered homozygous. This genetic configuration can influence the expression of traits, often leading to uniform characteristics in the phenotype, or observable traits, of the organism.

In contrast, having two different alleles for a trait would describe a heterozygous condition, not homozygous. The presence of one dominant and one recessive allele also indicates a heterozygous state since it involves two distinct alleles. Multiple alleles for a trait refer to more than two variations of a gene existing within a population, but an individual can only possess two alleles for any given trait. Thus, the definition clearly aligns with the concept of homozygosity as having identical alleles.