In a diploid organism, which statement accurately defines an allele?

• A. An allele is a regulatory element; a gene is a protein
• B. An allele is the same as a gene
• C. An allele is a copy of a chromosome
• D. An allele is a variant form of a gene

Answer: (D)

An allele is a variant form of a gene at a specific location on a chromosome. In a diploid organism, you have two copies of each chromosome, so you typically carry two alleles for each gene—these can be the same (homozygous) or different (heterozygous). Alleles arise by mutation and can lead to differences in the gene product or its regulation, influencing traits depending on how they interact (dominant vs recessive, etc.). This is distinct from a regulatory element, which is a DNA sequence that controls when or how much a gene is expressed, not a version of the gene itself. It’s also not a copy of a chromosome, and while a gene is the unit of inheritance, an allele is the specific variant form of that gene occupying the gene’s locus.