Perhaps one of the most powerful applications of markers is genetic mapping. This technology allows researchers identify markers located within or very close to genes of interest. With this it is possible to trace these genes through generations of crosses.

If the trait in question is complex and multigenic, mapping will also be of tremendous help to dissect the various genes or loci that contribute to the overall characteristic. Determining the amount of impact each locus delivers is a process called Quantitative Trait Loci or QTL mapping.

Single gene or QTL?

Determining whether a trait of interest is controlled by a single gene or by multiple loci is an important first step in genetic mapping. A general rule of thumb is that the stronger the heritability of the trait, the less complex (fewer loci) it encompasses. Another indication is the phenotypic data generated. Narrow distributions for specific phenotypes indicate single gene control whereas multigenic traits usually show a single, broader distribution of phenotypes.

Example: Disease Resistance

In either case, the key to successful mapping is generating a population that shows good segregation for the trait(s) of interest.

Association Mapping

In some cases, depending on the population being used, it might make more sense to take an Association Mapping approach. The following flowchart outlines the differences between linkage QTL analysis and association mapping: