QTL and markers

Some characteristics of an animal – horns / polledness, for example, as well as many hereditary defects – are controlled from one single location in the genome. With other traits, such as yield, size and longevity, many locations in the genome play a role, in addition to environmental influences. Such traits are called quantitative traits. The figure below shows the influence of individual chromosome regions on cow fertility (VanRaden et al, 2008). The higher the deviation the stronger the influence.

What can be clearly identified is that:

• All chromosomes influence fertility
• Some locations of the genome play a much bigger role than otherse

A location in the genome that significantly influences a quantitative trait is called a QTL (quantitative trait locus). The precise location of a QTL on the genome is generally not known. However, markers which are located in proximity to a QTL are more and more often becoming relatively easy to determine. 

However, a marker is only then of benefit if 

• It occurs in different variants (example M and m) and 
• One variant of the marker occurs more frequently with the positive variant (+) of the QTL

The nearer a marker is to a QTL, the bigger the chance that it can assume a representative role for the QTL in the genome analysis. The relationship between markers and QTL can be interrupted from generation to generation and mainly depends on the breed. 

SNPs are good markers as they occur frequently in the genome and their determination can be highly automated in the laboratory.