Understanding fundamental genetic terms is essential for grasping how living organisms inherit traits. A “locus” is a specific, fixed position on a chromosome where a gene or genetic marker resides. It is fundamental to studying how genetic information is organized and passed down.
The Chromosomal Address
A locus is not a physical structure visible to the naked eye or standard microscope. Instead, it functions as a conceptual “address” on a chromosome, similar to a street address.
Chromosomes are organized into arms, designated ‘p’ for the shorter and ‘q’ for the longer. Geneticists divide these arms into regions and bands, visible as light and dark stripes when chromosomes are stained.
These banding patterns serve as landmarks, allowing researchers to map and identify loci. For instance, a locus is described by its chromosome number, arm (p or q), and a series of numbers for the region, band, and sub-band, such as “7q31” for a gene on the long arm of chromosome 7. Genes are consistently found at fixed loci across individuals of a species.
What’s Within a Locus
While the locus defines a fixed location, the actual biological material occupying that spot is typically a gene. A gene is a segment of DNA that contains instructions for building specific proteins or regulating cellular processes, thereby influencing a particular trait. However, a gene can exist in different versions, known as alleles.
For example, a locus responsible for a certain characteristic might have one allele leading to one outcome and another allele leading to a different outcome for that same characteristic. Although the locus itself remains the consistent physical address on the chromosome, the specific allele residing there can vary between individuals. This variation in alleles at a given locus is a fundamental source of genetic diversity.
Loci and Genetic Variation
The presence of different alleles at specific loci is a primary driver of genetic variation within a population. This variation explains why individuals of the same species exhibit a wide range of observable traits, even though they share similar genetic blueprints.
Each individual inherits two copies of most chromosomes, one from each parent, meaning they receive two alleles for each locus. These two alleles, which can be identical or different, determine an individual’s genetic makeup for that specific characteristic.
Consider a simple example like eye color, where a specific locus on a chromosome contributes to this trait. Different alleles at this locus can result in variations such as brown, blue, or green eyes. The combination of alleles an individual possesses at this specific locus, along with other interacting loci, ultimately contributes to their unique eye color. This interplay of alleles at various loci forms the basis of inheritance, contributing to the rich diversity observed in nature.