Dairy Sheep Symposium - the Department of Animal Sciences ...

Identifying major genes or QTL related to economically important traits and selecting
animals based on genotype is an efficient tool to improve livestock production and product
quality. There are two main strategies designed to identify the genes underlying complex traits,
as are milk related traits; the linkage analysis in a whole genome scan and the association test
using candidate genes. These two strategies, which are described below, are now in their first
steps in dairy sheep breeding, but they will most probably be applied in deep in the near future.
QTL Detection in Dairy Sheep Using a Whole Genome Scan
This procedure is based on linkage mapping using genetic markers and segregating families.
A genetic marker is any polymorphic sequence, whose alleles can be distinguished from each
other, microsatellites being the most widely used markers in linkage studies. As QTLs could be
located anywhere, markers should be spread over the entire genome, so that at least 150 to 250
evenly spaced markers are required for a complete low-density genome scan. Segregating
families can be experimental crosses or outbred families.
Experimental Crosses
To map the genes underlying a specific phenotype an experimental cross is performed, by
mating divergent parental lines and using the resulting F 1 individuals to generate a large segregating
F 2 or a backcross population. The F1 animals show a high heterozygosity at marker loci
and, in particular, at those loci that account for phenotypic differences between the two populations.
This approach is used to identify the genes contributing to the differences observed for
phenotypic traits between two different breeds or divergent selected strains of a breed. This
strategy has been extensively used in pigs and cattle (Andersson et al., 1994; Brenneman et al.,
1996). In sheep we can cite experimental crosses for detecting complex traits, for example
between selected divergent lines for parasite resistance in New Zealand populations (Crawford et
al., 1997) or between Lacaune and Sarda breeds for milk production traits. An illustration of a
backcross is shown in figure 3.
Figure 3. Intercrosses between divergent populations. Example of a backcross between
two divergent sheep breeds
This approach offers several advantages. The cross between phenotypically divergent lines will
presumably generate an important allele substitution effect in F2 or backcross populations and the