Mapping of quantitative trait loci

Quantitative traits refer to the traits whose phenotypic characteristics vary in degree and can be attributed to the interactions between two or more genes and their environment. Though not necessarily genes themselves, quantitative trait loci (QTLs) are stretches of DNA that are closely linked to the genes that underlie the trait in question. QTLs can be molecularly identified (for example, with PCR) to help map regions of the genome that contain genes involved in specifying a quantitative trait. Mapping of QTLs requires use of many interdisciplinary approaches, and some of them will be discussed here.

Overview of QTL Mapping

The problem with analyzing polygenic traits like height is the fact that many genetic loci affect the trait. Hence, traditional techniques of mutational analysis cannot be solely employed to solve this question. [cite journal | author=Flint and Mott | title = [http://www.nature.com/nrg/journal/v2/n6/pdf/nrg0601_437a.pdf Finding the molecular basis of quantitative traits: Successes and Pitfalls] | journal=Nat.Rev.Gen | volume=2 | pages=437–445 | month=June | year=2001] The idea behind QTL Mapping is to determine the "degree of association" of a specific region on the genome to the inheritance of the trait of interest. If the association is high, there is a good probability that the region of genome we are looking at has something to do with expression of the trait.

Towards this goal, the first step in QTL Mapping is creation of a map of the genome using molecular markers like RAPD, RFLP, SNP etc. These markers serve as landmarks on the genome. The most ideal scenario is to have markers spread at regular intervals over the entire genome, but that is often not the case, Prior to the advent of molecular markers, visible polymorphisms and allozymes were used in this step. The idea is to look at the pattern of association of these markers with the inheritance of the trait. It must be noted that these molecular markers are not necessarily genes, and thus QTL Mapping ends up telling us the degree of association of a stretch of DNA with a trait rather than pointing directly at the gene responsible for that trait. Different statistical methods are used to ascertain whether the degree of association is significant or not.

uggested Readings

* [http://www.cs.cmu.edu/~epxing/CBML/linkage-qtl/qtl-broman.pdf Review of statistical methods for QTL mapping in experimental crosses]
* [http://www.nature.com/nrg/focus/multigen/reviews.html Nature Special Focus on Multi-factorial Genetics]

References