A locus (plural: loci, pronounced “low-sigh”) is a specific physical location on a chromosome where a particular gene or DNA sequence sits. Think of it as a street address for a gene. If you’ve encountered “loci” in a different context, like studying or memorization, it may refer to the “method of loci,” an ancient memory technique. Both uses share the same Latin root meaning “place,” but they come from very different fields.
Loci in Genetics
Every gene in your body has a fixed position on a specific chromosome, and that position is its locus. Because humans carry two copies of most chromosomes (one from each parent), you have two copies of each locus. The specific version of a gene sitting at that locus is called an allele. So a locus is the address, and an allele is who lives there. Your complete set of alleles across all your loci makes up your genotype, and the visible effects of those alleles (eye color, blood type, disease risk) make up your phenotype.
To see how this works in practice, consider the BRCA1 gene, which is involved in breast cancer risk. Its locus is on chromosome 17 at a position labeled 17q21, spanning roughly 81,000 base pairs of DNA. That address never changes from person to person. What varies are the alleles at that locus: some people carry a normal version, while others carry a mutation that significantly raises cancer risk. Mutations in a completely different gene, BRCA2, located at a different locus on chromosome 13, can independently cause the same disease. When mutations at different loci lead to the same condition, geneticists call this locus heterogeneity.
How Loci Help Map Traits
Most traits you can observe, like height, skin color, or susceptibility to a disease, aren’t controlled by a single gene. They’re influenced by many loci scattered across different chromosomes, each contributing a small effect. These are called quantitative trait loci (QTL), and scientists use statistical mapping techniques to identify which chromosomal regions are involved. QTL mapping helps researchers pinpoint the genetic neighborhoods that influence complex traits, factoring in interactions between multiple genes and the environment.
Geneticists also measure the distance between two loci using a unit called the centimorgan. One centimorgan equals a 1% chance that the two loci will be separated during the DNA-shuffling process that happens when sperm and egg cells form. Two loci that are close together on the same chromosome tend to be inherited together, while loci that are far apart get shuffled more freely. One important wrinkle: recombination rates vary enormously along a chromosome, so genetic distance between two loci doesn’t always match their actual physical distance in base pairs.
The Method of Loci: A Memory Technique
Outside of genetics, “loci” appears most often in the phrase “method of loci,” a memorization strategy developed in ancient Greece. The idea is straightforward: you mentally walk through a familiar place, like your house or your route to work, and “drop” each item you want to remember at a specific landmark along the way. To recall the information later, you retrace the route in your mind and “pick up” each item from where you left it. This is also called a “memory palace.”
The technique works because it hijacks your brain’s spatial navigation system. Brain imaging studies published in Science Advances show that using the method of loci activates the hippocampus and surrounding regions, the same areas your brain uses for navigating physical spaces, constructing mental scenes, and forming episodic memories (your personal, story-like recollections of events). By anchoring abstract information to spatial landmarks, you’re essentially encoding it through a system your brain is already very good at using.
Competitive memory athletes rely heavily on this technique. In training studies, participants who practiced the method of loci not only recalled more items but were also better at remembering the correct order of those items, since the sequence is baked into the route itself. The memory improvements persisted even after training ended, suggesting the technique produces durable changes in how the brain organizes and retrieves information.
Why the Same Word Appears in Both Fields
“Locus” comes directly from Latin, where it simply means “place.” Geneticists adopted it in the early 20th century to describe a gene’s place on a chromosome. The memory technique was already called the “method of loci” for centuries before that, referring to the mental places where you store information. In both cases, the core concept is the same: a defined location that helps you find something, whether that something is a gene or a grocery list.