2.4 Scientific and technological
advances
Developments in genetic improvement
methods
Quantitative genetics
A breeding scheme aims to achieve genetic
improvement in the breeding goal through the
selection of the animals that will produce the
next generation. The breeding goal reflects the
traits that the breeder aims to improve through
selection. The rate of genetic improvement
($G) with respect to the breeding goal (and the
underlying traits) depends on the amount of
genetic variability in the population, the accuracy
of the selection criteria, the intensity of selection,
and the generation interval.
Maintenance of genetic variation is a condition
for continuous genetic improvement. Genetic
variation is lost by genetic drift and gained by
mutation. Therefore, the minimum population
size to maintain genetic variation is a function
of the mutation rate (Hill, 2000). Selection
experiments in laboratory animals have shown
that substantial progress can be maintained for
many generations, even in populations with an
effective size well under 100, but that responses
increase with population size (ibid.).
The loss of genetic variation within a breed
is related to the rate of inbreeding (ΔF). In the
absence of selection, ΔF is related directly to
the number of sires and dams. In populations
undergoing selection, this assumption is no longer
valid because parents contribute unequally to
the next generation. A general theory to predict
rates of inbreeding in populations undergoing
selection has recently been developed (Woolliams
et al., 1999; Woolliams and Bijma, 2000). This
approach facilitates a deterministic optimization
of short and long-term response in breeding
schemes.