Alleles are best described as

• A. Different genes at different loci
• B. Identical forms of a gene
• C. Mutations in a gene
• D. Different forms of the same gene at a locus

Answer: (D)

Alleles are the variant versions of a gene found at the same place on homologous chromosomes. For each gene, there is a specific location called a locus; at that locus, the sequence can vary, giving different forms of the gene. These alternate forms are what we call alleles. In a person, you usually have two alleles for a gene—one from each parent—so you can be homozygous (two identical alleles) or heterozygous (two different alleles). An everyday example is the gene that determines blood type, which has several alleles at one locus, producing different glycoprotein patterns on the surface of red blood cells. Describing alleles as different forms of the same gene at a locus captures this idea precisely. The other options describe different concepts (different genes, mutations, or identical copies) and don’t fit the definition of alleles.