What is a locus?

• A. The function of a gene
• B. The location of a gene on a chromosome
• C. The complete set of genes
• D. A sequence of amino acids

Answer: (B)

A locus is the fixed position of a gene on a chromosome. This specific spot is where the gene resides, and it’s the same location on homologous chromosomes, which is essential for tracking inheritance and for genetic mapping. The idea helps explain why we talk about alleles occupying the same locus on each chromosome and how crossing over and segregation affect inheritance. The other ideas describe different things: a gene’s function is what the gene does, the complete set of genes is the genome, and a sequence of amino acids is a protein’s primary structure, not where a gene sits on a chromosome.