What is an allele?

• A. A nucleotide
• B. A variant form of a gene
• C. A chromosomal region
• D. A type of protein

Answer: (B)

An allele is a variant form of a gene. It represents a different DNA sequence at the same gene locus on a chromosome, and these sequence differences can lead to variation in the trait that the gene controls. In diploid organisms, you typically have two alleles for each gene—one on each homologous chromosome. They can be the same (homozygous) or different (heterozygous). Some alleles are dominant and others recessive, influencing how traits appear in the organism’s phenotype depending on which alleles are present. Alleles arise by mutation and there can be many different ones in a population, providing genetic diversity. The other options don’t fit because a nucleotide is a single building block of DNA, a chromosomal region is a larger stretch of DNA that may contain multiple genes, and a protein is the product made from gene expression, not the gene variant itself.