An organism with two different alleles for a trait is called what?

• A. Homozygous
• B. Allele
• C. Heterozygous
• D. Genotype

Answer: (C)

Having two different versions of a gene at the same location on homologous chromosomes means the organism is heterozygous for that trait. An allele is a variant form of a gene, and the genotype is the pair of alleles present. When those two alleles are different, the genotype is described as heterozygous. This contrasts with homozygous, where the two alleles are identical (such as AA or aa). For example, an Aa plant is heterozygous for the trait, whereas AA or aa are homozygous. The phenotype—the observable trait—may show the dominant version even though the genotype is heterozygous, but the important label for two different alleles is heterozygous.