Nicholas Barton and Peter Keightley, authors of “Understanding Quantitative Genetic Variation”, explain the current and potential use of identifying quantitative trait loci (QTL)—portions of DNA that cause trait variation. Typically, QTLs underlie continuous traits such as height or body weight. In addition, many QTLs are associated with a single trait as many genes usually determine a single phenotypic trait. QTLs are vital in the field of genetics. Knowing the number of QTLs that explain variations in the phenotypic trait determines the genetic architecture of a trait. However, various biases exist in the distribution of QTL effects. Barton and Keightley provide several factors that clarify why it is difficult to estimate the actual numbers and effects of loci that influence a quantitative trait.

These biases include missing closely linked QTL with opposite effects, having undetected QTL that reside below the lower limit, mistaking closely linked QTL with effects in the same direction for a single QTL of larger effect, and the ‘Beavis effect’—the inflation of the estimated effects when samples are not large enough. These factors provide a remarkable insight into our knowledge and lack of knowledge of genetic variation. They result in an overall underestimate of the numbers of QTL and an overestimate of their effects and need to be noted when analyzing QTLs.

For more information on QTL, see Barton, N. and P. Keightley. 2002. “Understanding Quantitative Genetic Variation” by Barton and Keightley, Nature Reviews: Genetics 3: 11-21.