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

genetic predisposition for acquiring a disease, then environmental conditions, including standard
disease-prevention methods may be only partly effective in preventing disease. An oftenoverlooked
alternative approach to standard disease control methods would be selective breeding
to increase disease resistance in livestock.
Genetic resistance to disease involves many facets of the body’s defense system and their
interactions and is extremely complex. However there are two cases where the selection has done
first results in bacterial diseases as footrot (Raasda, 2000), parasite resistance (Crawford et al.,
1997; Coltman et al., 2001). As indicated above there are many investigations about mastitis
resistance and SCC as selection criteria in dairy sheep. In the next years further clarifying results
will elucidate this situation.
Particularly interesting regarding genetic resistance is scrapie an infectious disease of sheep
in which the infectious particle appears to be a particular form (scrapie prion) of a protein
molecule found in normal, healthy sheep (prion protein). Current experimental evidence strongly
suggests that there are prion protein forms that do not undergo the structural transformation to
scrapie prions. These prion proteins differ from those that easily convert to become scrapie
prions by single amino acid substitutions. For example, an arginine (R) at amino acid 171 of the
prion protein appears to prevent the prion molecule from undergoing the structural change
associated with strain C scrapie. In a similar manner, an alanine (A) at amino acid 136 appears to
prevent the prion molecule from undergoing the structural change associated with strain A
scrapie. Analysis at DNA level can easily determine the resistant/susceptible status of an animal
(Hunter, 2000). This knowledge makes entirely possible introducing in a breeding program
selection of animals for resistance to spongiform encephalopathy, so that in several generations
scrapie can be eliminated from the population (Hunter, 2000). There are many dairy breeds that
are starting a genotyping program of elite rams and in the next years, only resistant animals will
be eligible as AI sires.
In the next future with the help of molecular techniques, results in this area will be very
important in sheep breeding. The improvement of farm animals by selective breeding is a highly
effective and sustainable means of improving livestock. Genetic change is cumulative and
permanent, so that an improvement gained is maintained without further input. New genetic gain
also builds upon past improvement.
New Developments in Reproductive Techniques
Reproductive technologies are of high importance given the constraints of reproductive
abilities in any animal breeding schemes.
It is difficult to evaluate the future developments of reproductive technologies in animal
breeding programs, but recent advances such as fiber optics and in vitro technologies offer new
approaches for animal breeding and can assist in addressing problems of infertility or reduced
fertility in humans. Modern reproductive technologies include artificial insemination, semen
freezing and sexing, embryo transfer and embryo micromanipulation. Furthermore, these techniques
have been applied to produce animals that incorporate new genes, which are beneficial to
agriculture and to animal and human health. Reproductive technology is a complex item and we
will describe here some milestones that can improve animal breeding in the next future.