quantitative trait locus
A quantitative trait locus, usually shortened to QTL, is a stretch of the genome that statistical evidence ties to differences in a measurable trait. Think of it as a neighbourhood on a chromosome that the data flag as worth investigating because individuals carrying different versions of that region tend to have different trait values.
Researchers find QTLs by scanning the genome for spots where genetic markers travel together with the trait across a family or population. A marker that consistently lands more often in tall plants than short ones points to a nearby variant that influences height. Because the signal comes from a region rather than a single base, a QTL is an address, not yet the actual culprit gene — fine-mapping is needed to narrow it down.
Most quantitative traits are governed by many QTLs, each explaining only a slice of the variation, and their individual effects are often small. The honest caveat is that a QTL is a correlation in a particular population and environment: its apparent effect can shrink, shift, or vanish in a different setting, which is why replication across studies matters so much.
In tomato breeding, a famous QTL on chromosome 2 boosts fruit size; once located, breeders could select for the favourable version without waiting to grow and weigh every fruit by hand.
A mapped region lets breeders select indirectly.
In humans, genome-wide association studies are the dominant way to map QTLs; the variants they flag are often regulatory rather than protein-coding, and pinning down the causal gene remains the hard part.