What is an allele?

• A. The specific form of a gene.
• B. A complete chromosome.
• C. A different chromosome.
• D. A regulatory region of a gene.

Answer: (B)

An allele is a variant form of a gene at a specific location on a chromosome. Genes are the units that code for traits, and at the same locus on homologous chromosomes you can have different versions of that gene. Those versions are the alleles. They can differ in DNA sequence, which can change the protein produced or how the gene is regulated, leading to variation in the trait.

In a diploid organism you usually have two alleles for each gene—one from each parent. If both alleles are the same, you’re homozygous for that allele; if they’re different, you’re heterozygous.

The other options describe larger genomic pieces or different regions, not a specific gene variant: a complete chromosome is a whole chromosome, a different chromosome is another chromosome, and a regulatory region is DNA that controls gene expression, not the variant form of the gene itself.