Gene locus is defined as

• A. A chromosome segment coding tRNA exclusively.
• B. The start point of replication.
• C. The collection of all genes.
• D. The position of a gene on a chromosome.

Answer: (D)

A gene’s locus is the exact position on a chromosome where that gene is located. This fixed location is what geneticists use to map genes, compare them across individuals, and study inheritance, since each allele occupies the same locus on homologous chromosomes. It isn’t restricted to tRNA genes—any gene has a locus. It isn’t the starting point of DNA replication (the origin of replication), and it isn’t the collection of all genes (the genome). So the locus uniquely defines where a gene sits on the chromosome.