Which statement best defines an allele?

• A. The visible trait of an organism
• B. A chromosome carrying multiple genes
• C. A variant form of a gene
• D. The entire genetic material

Answer: (C)

An allele is a variant form of a gene, occupying the same position (locus) on homologous chromosomes. Differences in the DNA sequence at that gene create these alternative versions, which can lead to different forms of the trait. In a diploid organism you typically have two alleles for each gene, which may be the same or different, and their interaction (dominant versus recessive) determines how the trait is expressed. This distinguishes alleles from the visible trait itself (the phenotype), from a chromosome that carries many genes, or from the entire genome.