- A genetic locus is a fixed, specific position on a chromosome where a gene or another segment of DNA is located. The term comes from the Latin word locus, meaning “place,” and it emphasizes the idea of a precise address within the genome. A locus may refer to a single nucleotide, a gene, or even a non-coding region that plays a regulatory role. Importantly, every individual of a species has the same set of loci arranged along their chromosomes, but the DNA sequences at those loci can differ between individuals, giving rise to genetic variation.
- Within a locus, the sequence that is actually present can exist in multiple forms, known as alleles. For example, at a locus associated with flower color in a plant species, one allele might code for red pigment, while another allele codes for white pigment. Thus, the concept of a locus provides the framework for discussing where in the genome allelic differences occur. While the locus is constant across a population, the alleles at that locus may vary, and this variation is the basis of genetic diversity, inheritance, and evolution.
- A locus can contain more than just protein-coding genes. It might represent regulatory regions, such as promoters or enhancers, non-coding RNAs, or even structural elements of chromosomes like centromeres. Because of this, the study of loci is not limited to understanding genes alone but extends to any functional or structural DNA segment. Moreover, loci can be defined at different levels of resolution: a broad chromosomal band visible under a microscope, a single gene identified through sequencing, or even a single nucleotide polymorphism (SNP) that marks variation between individuals.
- The concept of genetic loci has practical importance in research and medicine. In classical genetics, loci were identified by tracking inheritance patterns of traits through pedigrees. With modern genomics, loci are mapped with high precision using sequencing technologies. This has enabled genome-wide association studies (GWAS), where researchers search across thousands of loci to find variants linked to diseases or traits. For instance, a GWAS may reveal that a specific locus on chromosome 9 is associated with heart disease risk. While the locus identifies where the variation lies, further investigation often determines how the allelic differences at that locus influence the trait.
