Alleles are the different forms of what genetic unit?

• A. A trait that is recessive
• B. The same allele in two individuals
• C. Different versions of a chromosome
• D. Different forms of a gene

Answer: (D)

Alleles are different versions of a gene. A gene is a specific DNA sequence at a particular place on a chromosome that influences a heritable trait. Small sequence differences in that gene create alternate forms—alleles—that can change how the trait appears or functions. Because genes sit at the same position on homologous chromosomes, an individual typically carries two alleles for each gene. The other options describe a trait, a chromosome variant, or confuse the unit with the chromosome itself, but they don’t define alleles as gene variants. For example, a gene for coat color can have alleles that lead to different colors.