What term describes a condition of only having one gene variant for a trait?

The condition of having only one gene variant for a trait is accurately described by the term "homozygous." When an organism is homozygous for a particular trait, it has two identical alleles for that trait, whether they are dominant or recessive. This means that, for that specific gene, both parents contributed the same variant.

In contrast, the term "heterozygous" refers to an organism that has two different alleles for a specific gene, which does not fit the description provided in the question. The terms "phenotypic" and "genotypic" describe different concepts; phenotypic refers to the observable traits of an organism, while genotypic refers to the genetic makeup itself, but does not specify the number of gene variants. Hence, the most accurate term for having one gene variant is "homozygous."