How would you define an allele?

• A. An allele is a complete set of genes in an organism
• B. An allele is a variant form of a gene
• C. An allele is a type of chromosome
• D. An allele is a protein produced by a gene

Answer: (B)

An allele is best defined as a variant form of a gene. Genes are segments of DNA that code for specific traits, and they exist in multiple forms due to variations in their nucleotide sequences. These variations can result in different expressions of a trait, such as flower color in plants or blood type in humans.

When a gene has different forms, each form is referred to as an allele. For example, the gene that determines eye color has several alleles that might code for blue, brown, or green eyes. The presence of different alleles in the genotype of an organism contributes to the diversity of traits observed within a population.

In contrast, a complete set of genes in an organism refers to the organism's entire genome, and not just individual alleles. A chromosome is a structure that organizes and carries genes, rather than being an allele itself. Finally, proteins are products of gene expression, but they are not alleles; rather, they are the result of the translation of genes, which alleles provide the variations for. Thus, the definition of an allele focuses specifically on its role as a gene variant.