What is a locus?

What is a locus?

• A. The location on a chromosome where a gene is located; each locus has a pair of genes (one on each chromosome).
• B. The total number of genes on a chromosome.
• C. A protein that regulates gene expression.
• D. The central region of a chromosome.

Answer: (A)

A locus is the fixed position on a chromosome where a gene or genetic marker is located. In a diploid organism there are two copies of that position, one on each homolog, which may carry different alleles. This positional concept is what genetic mapping and inheritance rely on, since alleles at the same locus segregate during meiosis and can be separated or linked depending on distance between loci. The other options describe different ideas: the total number of genes on a chromosome refers to gene count, a protein that regulates gene expression is a regulatory protein, and the central region of a chromosome is the centromere.

A locus is the fixed position on a chromosome where a gene or genetic marker is located. In a diploid organism there are two copies of that position, one on each homolog, which may carry different alleles. This positional concept is what genetic mapping and inheritance rely on, since alleles at the same locus segregate during meiosis and can be separated or linked depending on distance between loci. The other options describe different ideas: the total number of genes on a chromosome refers to gene count, a protein that regulates gene expression is a regulatory protein, and the central region of a chromosome is the centromere.