Introduction
Substantial advances have been made over the past
decades through the application of molecular genetics in the identification of loci and chromosomal regions
that contain loci that affect traits of importance in
livestock production (Andersson, 2001). This has enabled
opportunities to enhance genetic improvement
programs in livestock by direct selection on genes or
genomic regions that affect economic traits through
marker-assisted selection and gene introgression
(Dekkers and Hospital, 2002). To this end, many theoretical
studies have been conducted over the past several
decades to evaluate strategies for the use of molecular
genetic information in selection programs. The
extra responses to selection that have been predicted
by several studies (e.g., Meuwissen and Goddard,
1996) have resulted in great optimism for the use of
molecular genetic information in industry breeding
programs. Objectives of this paper are to assess the
extent to which and in which ways marker and gene
information has been used in commercial livestock improvement
programs, to assess the successes and limitations
that have been experienced in such applications,
and to discuss strategies to overcome these limitations.
I will start with a discussion of the principlesof the use of molecular genetic information in genetic
improvement, which will set the stage for the analysis
of marker-assisted selection in commercial breeding
programs.