The definition of quantitative trait locus (QTL) is exactly what you would expect if you break down the term. A trait is something that distinguishes people from one another. Traits can be as simple as eye color or fairly complicated, such as how anxious a person is in their every day life. Lets think about this for a moment. Eye color has discrete values such as blue, brown, green, etc. but defining anxiety is a little more complicated. What if we define anxiety numerically, for example with a one to ten scale. One being that cool cousin that never worries about anything and ten being that crazy aunt that frantically runs around the house every holiday. Obviously there are all kinds of varitions in anxiety of people in between. Do you see how anxiety can be a quantitative trait? It is continuous and is not managed by a pool of discrete values.

Now what about the locus aspect of QTL. We know that locus is plural for loci and refers to some sort of position. Loci actually refers to the position of a single gene on the chromosome. So theoretically if the trait eye color was due to a single gene and we were interested in its position on a chromosme we would call this a trait loci.

Now if we go back Since we are talking about a quantitative trait, several genes may be involved and located on several spots on the chromosome, hence locus.