Which term describes organisms with unlike gene pairs?

The term that describes organisms with unlike gene pairs is heterozygous. Organisms that are heterozygous carry two different alleles for a particular trait, meaning they have inherited different versions of a gene from each parent. This genetic variation can lead to a diverse range of traits within a population, playing a critical role in evolution and adaptation.

In contrast, homozygous refers to organisms that have identical alleles for a trait, meaning both inherited genes are the same. Monomorphic denotes populations where all individuals have the same allele at a particular gene locus, indicating a lack of genetic diversity. Polygenic refers to traits that are controlled by more than one gene, resulting in continuous variations, such as height or skin color, rather than a focus on the presence of unlike gene pairs. Thus, heterozygous specifically highlights the genetic difference in alleles, making it the correct choice for this question.