What is the definition of an allele?

An allele is defined as a variant form of a gene. Genes exist in multiple forms, which can vary in sequence and lead to different traits or characteristics being expressed in an organism. For example, a gene that determines flower color may have different alleles that produce red, yellow, or white flowers. This concept is fundamental in genetics because it explains how genetic variation contributes to diversity within a species.

The other options denote different aspects of genetics but do not accurately define an allele. A different combination of genes refers to the genotype of an organism rather than a specific form of a gene. A type of chromosome describes the structure that contains the genetic material rather than a specific variant of a gene. A complete set of DNA typically refers to the entire genome of an organism, not just the individual variants of genes. Consequently, the definition of an allele distinctly encapsulates the idea of variation at the gene level.