locus
Meaning of locus: An address rather than a resident: the locus says where on the chromosome to look, and the allele says what is found there.
Definition of locus
A locus is the fixed position of a gene or other identifiable DNA sequence on a chromosome, written as the chromosome number, the arm and the band. It names a location, not the sequence occupying it. The alternative versions that can sit at one locus are its alleles. The plural is loci, and the word keeps its plain Latin sense of place elsewhere in medicine.
How is a locus written?
| Part of the address | What it identifies |
|---|---|
| Number | Which of the numbered chromosome pairs |
| Letter p or q | The short arm or the long arm, measured from the pinched center |
| Region, band and sub-band | The position along that arm, counted outward from the center |
The bands are the pattern of light and dark stripes that appears when a chromosome is stained and viewed under a microscope, so the numbering marks landmarks rather than measured distances. Read outward from the center, an address of this kind lets two laboratories on opposite sides of the world describe the same place in the same words. Modern sequencing gives a far finer coordinate, counting bases along the chromosome, but the band notation survives because it is readable and because it corresponds to what can actually be seen.
How does a locus differ from a gene or an allele?
A gene is a sequence that does something. A locus is where a sequence sits. An allele is one of the versions that can occupy that locus in different people. Two people carrying different alleles at the same locus is the ordinary state of affairs, and it is what genetic testing reports on. Keeping the three apart makes several ideas expressible. Locus heterogeneity describes one condition arising from faults at entirely different loci, which is common and explains why a negative result at one locus does not close the question. Linkage describes loci close enough together to be inherited as a package. A locus can also be occupied by something that is not a gene at all, such as a regulatory sequence or a repeated stretch used as a marker.
Where else does the word appear?
Outside genetics it keeps the plain meaning. The locus coeruleus is a small cluster of nerve cells in the brainstem, named for the blue tint of its pigment and for its position, and it supplies much of the brain's noradrenaline. The word is also used for a defined point on the body surface, including an acupuncture point, and in psychology for the locus of control, meaning where a person believes the causes of events lie. In every one of these the sense is identical: a defined place, with the surrounding field supplying what occupies it.
Used in a sentence
The mutation was mapped to a specific locus on the chromosome.
Good to know
The word locus takes on a specific technical meaning in genetics, denoting the fixed chromosomal position of a sequence rather than the sequence itself, which differs from its more general use elsewhere in medicine to describe simply where something is located.
Word origin
Where “locus” comes from
From Latin locus, meaning place.