How can molecular genetics help?
Molecular genetic analyses of quantitative traits lead to
the identification of two broadly different types of
genetic loci that can be used to enhance genetic
improvement programmes: causal mutations and presumed
non-functional genetic markers that are linked
to QTL (indirect markers). Causal mutations for quantitative
traits are hard to find, difficult to prove and few
examples are available1
. By contrast, non-functional or
anonymous polymorphisms are abundant across the
genome and their linkage with QTL can be established
by evidence of empirical associations of marker genotypes
with trait phenotype. Two approaches are used to
identify indirect markers1
: directed searches using candidate-gene
approaches in unstructured populations6
;
and genome-wide searches in specialized populations,
such as F2 crosses. Because candidate-gene markers
focus on polymorphisms in a gene that are postulated
to affect the trait, they are often tightly linked to the
QTL. A candidate-gene marker can occasionally represent
the functional variant itself, although this is difficult
to prove1
. Genome scans, conversely, can only identify
regions of chromosomes that affect the trait. The length
of these regions is typically 10–20 cM, but the exact
position and number of QTL in the region is unknown.