Which term describes a variation of a gene located at a locus?

• A. Genome
• B. Trait
• C. Genotype
• D. Allele

Answer: (D)

An allele is a variation of a gene that occupies the same spot, or locus, on homologous chromosomes. Genes sit at defined loci, and different versions of a gene in that spot are called alleles. In an individual, you typically have two alleles for a given locus—one on each chromosome of the pair—and these can be the same or different. The specific combination of alleles can influence the trait you express, often with environmental factors also playing a role. For example, a gene for color might have a purple allele and a white allele at the same locus. The genome refers to the entire set of genetic material, a trait is an observable characteristic, and genotype describes the pair of alleles present at that locus. So, the variation of a gene located at a locus is called an allele.