genetic marker
Meaning of genetic marker: A signpost on a chromosome, useful because its position is known, whether or not it does anything at all.
Definition of genetic marker
A genetic marker is a stretch of DNA at a known location on a chromosome that differs between individuals in a way that can be measured. Its usefulness lies in being identifiable and mappable rather than in what it does. Markers are used to follow inheritance through a family, to tell one person's sample from another, and to home in on genes behind a trait or a disease.
Does a marker have to cause anything?
No, and most cause nothing whatever. Chromosomes are handed down in long blocks rather than letter by letter, so a marker lying close to an important gene travels with it through the generations. Following the marker therefore follows the gene, even when nobody yet knows which gene it is or what it does. That trick located many disease genes before reading whole genomes became routine. It also explains a limitation that gets lost in reporting: a marker found more often in people with a condition may simply be a neighbor of the variant that matters, so the finding points at a stretch of chromosome rather than at a cause.
What kinds are used?
| Type | What varies | Common use |
|---|---|---|
| Single letter variant | One letter of the DNA sequence | Scanning whole genomes for association |
| Short repeat | How many times a short sequence repeats | Identity and parentage testing |
| Cut-site variant | Whether an enzyme can cut at that point | Older family linkage studies |
| Copy number variant | How many copies of a segment are present | Studying larger structural differences |
How is it different from a diagnostic gene test?
A diagnostic test reads a gene already known to cause a condition and reports whether the causative change is present. Genetic testing of that kind answers a question about one person. A marker answers a question about position, and can do so without any knowledge of function, which is why markers underpin forensic identification, parentage work and ancestry studies as readily as medicine. A molecular marker in the wider laboratory sense is different again, since it may be a protein or another measurable substance rather than a piece of DNA. Where markers do carry health meaning, they describe a genetic predisposition shared across a group rather than a verdict about an individual.
Also known as
Used in a sentence
Genetic markers were used to trace the affected chromosome region through the family.
Good to know
A marker linked to a disease is often a neighbor of the responsible variant rather than the cause of anything, so an association identifies a region of a chromosome rather than a mechanism.
Word origin
Where “genetic marker” comes from
Genetic comes from the Greek genesis, meaning origin or birth, by way of genetikos, relating to origin.