In this study the effect of artificial
selection on BLUP EBV for production traits on the
allele frequencies of a pleiotropic QTL affecting both
production and disease susceptibility was investigated.
Stochastic simulations were used to model artificial se-
lection on a production trait that is controlled, in part,
by a biallelic QTL that also controls susceptibility to
disease. The QTL allele increasing production also in-
creased susceptibility to disease. Different modes of ac-
tion and proportions of variation accounted for by the
QTL were assessed for the production trait. The main
results indicated that alleles that confer susceptibility
to the disease could be maintained in the population
over a long period, depending on the mode of action of
the QTL. In addition, the results of the study indicate
that, under various conditions, it is possible to find
pleiotropic QTL that control 2 traits despite these traits
appearing to be uncorrelated. Therefore, in practice, an
estimate of the genetic correlation between 2 traits may
be misleading when the presence of such a QTL exists.
The results of this study have implications for breeding
programs. For example, if a pleiotropic QTL exists that
favors heterozygotes for a production trait, it would be
very difficult to remove disease susceptibility alleles via
traditional selection methods.
fitness, health-related trait, pleiotropy, quantitative trait locus, selection
©2009 American Society of Animal Science. All rights reserved.
J. Anim. Sci. 2009. 87:850–859
In domestic livestock species, improving health-relat-
ed traits has become increasingly important (Bishop et
al., 2002). To evaluate the benefits of including health-
related traits in the breeding objective, it is necessary
to know their genetic relationships with other traits of
importance such as production traits. It is generally ac-
cepted that if the estimated additive genetic correlation
between production and disease susceptibility is zero,
improvement in production traits can be accomplished
without adversely affecting the disease trait (Falconer
and Mackay, 1996).
Most QTL mapping experiments have focused on pro-
duction traits, although QTL for health-related traits
may offer more benefits (e.g., Dekkers, 2004). Mapping
studies aimed at finding QTL for these low heritabil-
ity traits could easily have production data available,
allowing the search for pleiotropic QTL to be used in
marker-assisted selection programs.