What do you call an organism that has two identical alleles for a trait?

An organism that has two identical alleles for a trait is referred to as homozygous. This means that both alleles, one inherited from each parent, are the same for that particular gene. For example, if an organism has two alleles for flower color that are both “RR” (for red) or “rr” (for white), it is homozygous for that trait. This genetic uniformity often leads to a consistent expression of that trait in the organism.

In contrast, heterozygous refers to an organism that has two different alleles for a trait, which would introduce variability in the expressed characteristics. "Multiple alleles" and "polygenic" describe different concepts where multiple alleles can exist for a trait or where multiple genes control a single trait, respectively, but they do not define the concept of having two identical alleles. Thus, the correct term for having identical alleles is indeed homozygous.