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

Outbred Population
The objective in this case is to map the QTL that are underlying the genetic variance observed
for traits in a commercial population. These studies profit from two specific features of dairy livestock
populations: (1) the development of artificial insemination and semen preservation has made large
half-sib families with a common antecessor available. (2) The existence of breeding programs
allow for the use of production values corrected for the environmental factors rather than individual
production records, the former values showing less complexity due to the reduction of non-genetic
noise. Two main schemes, the “daugther design” and the “grand-daughter design” are used for this
purpose (Weller et al., 1990).
In the “daughter design” a single sire can have hundreds of descendants with records on different
quantitative traits (figure 4). The daughters of a sire heterozygous for a marker are genotyped for a
marker and recorded for the quantitative traits. If alternative groups of daughters, sorted by marker
allele, shows differences in the quantitative traits indicate a linkage between the marker and a QTL
underlying genetic variation on the trait. The use of genetic markers spanning whole genome (genome
scan) will indicate the traits segregating for quantitative traits in one family, analysis of a number of
families will provide useful information about the genetic variability on a trait in a breed.
Figure 4. Half-sib families and QTL mapping. Illustration of a daugther design
An alternative analysis is the so-called granddaughter design. In the ‘granddaughter design’
(Weller et al, 1990), the basic idea is to reduce the residual variability of performance. Here the trait
is not measured directly on the progeny but it is estimated as the mean performance of granddaughters.
The design involves 3 generations as is shown in figure 5. Only generations 1 and 2 are genotyped,
whereas performances are measured only at generation 3. For a similar detection power, the number
of marker genotypes here is usually threefold lower than in a ‘daughter design’.
Daughter and granddaughter designs have been used mainly in dairy cattle with excellent results
(Georges et al., 1995; Coppieters et al., 1998; Ron et al., 2001). In dairy sheep these investigations
show larger limitations when compared with dairy cattle. The different production systems applied
in each local breed imply lower population sizes with relatively recent implantation of selection
schemes. These circumstances does not permit, in most of the cases, a granddaughter design, and
the limited size of half-sib families in a daughter design results in a low statistical power of the
study. However, many experiments are currently being undertaken searching for QTL in dairy
sheep and first results have already been reported (Diez-Tascón et al., 2001).