QTL: Quantitative Trait Locus

What's a QTL?
asks KW, of Edmonton, Alberta, Canada.

People are different. But not all differences are the same! Some traits, such as gender is binary. Outside of certain fetish circles, you're one or the other. It's also determined, on the most part, by a single gene. Some traits, are much more complex.

A QTL is a locus contributing to a quantitative trait. So for an example, height is a quantitative trait. It's continuously variable in humans, not like gender, which is one or the other. Many genes contribute to height, like ones controlling growth hormones, their receptors, genes involved in metabolism of nutrients and bone/muscle, etc. You can map where genes are that contribute to this quantitative trait, by studying lots of people of varying heights, and collecting information about their genetic variation, and try to correlate which variations are associated with difference in height. Those variations may not be the actual determining factor for height, but they're close, in terms of distance on DNA sequence. So a collection of these differences will narrow down a region of the genome (refered to as a locus) that may contain a gene or genes contributing to height. This region is called a quantitative trait locus; a locus that contributes to a quantitative trait.

The interest is in terms of disease. There are lots of cases where a gene for a disease has been identified. Unfortunately, they're usually caused by a single gene (such as Huntington), which is way easier to map since the genetics are so simple. But way more people are afflicted with complex diseases (heart disease, cancer, obesity, drug reactions) which are due to many genes, and are continuously variable like height. So there is a great interest in methods to identify QTLs, such as high throughput genotyping and statistical methods to use that data.