Which of the following best describes the term allele?

• A. The gene produced by insertion of a DNA fragment from another species
• B. The same gene found on all chromosomes of a species
• C. One of a pair of genes that occupies a specific site on a given chromosome
• D. Different DNA fragments found on the same chromosome

Answer: (C)

Allele is a variant form of a gene that occupies a specific locus on a chromosome. In diploid organisms, two homologous chromosomes carry the same gene at that site, and each can have a different allele, allowing genetic variation and the possibility of dominant or recessive traits influencing phenotype. The other statements describe concepts that aren’t about variant forms of a gene at a defined location: one refers to a gene from another species, another to an identical gene on all chromosomes, and another to different DNA fragments on a chromosome, none of which capture the idea of variant versions at a specific locus.