What is the definition of an allele?

• A. A chromosome without genes
• B. A variant form of a gene at a locus
• C. A type of antibody produced after exposure
• D. A protein that binds antigens

Answer: (B)

An allele is a variant form of a gene at a specific location (locus) on a chromosome. For each gene, organisms typically have two alleles—one inherited from each parent—so the two copies can be the same or different. These allele variants can change the gene’s product or how it’s regulated, which is why different alleles lead to variation in traits, such as the expression of blood group antigens. The other statements describe something unrelated to the genetic concept: a chromosome without genes, an antibody produced after exposure, or a standalone antigen-binding protein, none of which define what an allele is.