Genetic Selection and Transfer Pt 1

Genetic Selection and Transfer Objectives  Investigate genetic selection methods.  Develop a logical argument for cloning.  Distinguish between embryonic cloning and nuclear transfer. Genetic Selection •Permanent improvements in domestic animals can be made by genetic selection through natural or artificial means. •Natural selection occurs in wild animals, while artificial selection is planned and controlled by humans. Genetic Selection •Animals that exhibit desirable traits are selected and mated. •Animals that exhibit undesirable traits are not allowed to reproduce or are culled from the herd. Photo by Peggy Greb courtesy of USDA Agricultural Research Service. Genetic Selection •The goal of selection is to increase the number of animals with optimal levels of performance, while culling individuals with poorer performance. •Genetic improvement is a slow process and can take several generations to see an improvement in a trait. Genetic Selection Artificial insemination and embryo transfer are breeding methods that are commonly used to decrease the time taken to improve a trait. “Angus surrogate mother nurses her Romosinuano embryo transfer calf. Initially, scientists are investigating the influence of surrogate breed on Romosinuano calf traits such as length of gestation and birth and weaning weights” (USDA-ARS) Photo by Scott Bauer courtesy of USDA Agricultural Research Service. Genetic Selection •Traits are passed from parents to offspring, but some traits are more heritable than other traits. •That is, the genotype of an individual will be expressed more strongly and environment will be less influential for particular traits. Heritability of Various Traits in Livestock Trait Sheep Swine Cattle Weaning weight 15-25% 15-20% 15-27% 20-30% 20-30% 40-50% Post-weaning gain efficiency Post-weaning rate of gain 50-60% 25-30% 50-55% Feed efficiency 50% 12% 44% Loin eye area 53% 53% 56% Genetic Selection •Several genes influence some traits. •For example, rate of growth is a trait that is influenced by appetite, energy expenditure, feed efficiency, and body composition. Genetic Selection •Breeding systems aim to improve a single trait or multiple traits. •Single trait selection – aimed at improving one trait in a breeding program with little or no regard for improvement in other (associated) traits. Genetic Selection •Multiple trait selection – aims to simultaneously improve a number of traits. •Theoretically, multiple trait selection should result in a faster rate of gain toward a specific objective. Genetic Selection •Most domestic species now have a recognized system in place that allows breeders to estimate the genetic merit of individuals. •In the United States, cattle, sheep, goat, and swine breeders use expected progeny differences (EPDs). Genetic Selection EPDs are used to compare animals from the same species and breed. “Newly developed EPDs (expected progeny differences) make it possible to select for tenderness and carcass and beef quality traits in Brahman cattle, shown here at the ARS Subtropical Agricultural Research Station in Brooksville, Florida” (USDA-ARS). Photo by David Riley courtesy of USDA Agricultural Research Service. Genetic Selection For EPD values to be used effectively, one needs to know the breed averages, the accuracy of the EPDs, and who estimated the EPDs. A high EPD is not necessarily good; it depends on the trait being considered and breeding objectives.