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ch2notes - Chapter 2 How Are Animal Populations Improved...

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Unformatted text preview: Chapter 2 How Are Animal Populations Improved? Purpose of Animal Breeding is to improve populations not individuals. Tools of Animal Breeding (Decisions): Selection and Mating Selection - Moves the Population - Results in long-term change - two types: - Natural - Artificial - Under Human Control Artificial selection has two aspects: - Replacement Selection - Culling Goal of animal breeding is to: Let animals with best genes reproduce, next generation ( on average) has better genes. Over many generations, frequency of the desired genes in the population increase. Phenotypic Selection - Mass Selection Rank animals for trait of interest, pick the best ones. Underlying Assumption - The ones that perform the best have the best genes. What is the relationship between genotype and phenotype? Heritability (h2) When h2 is high, phenotypes are good indicators of underlying breeding value (genotype). When h2 is low, phenotype is less useful is predicting genotype, and phenotypic selection is less effective. Measuring Performance You can’t select on/change what you don’t measure! Selection requires Performance Testing. - Collecting data on all traits that are economically important. - Used in all livestock species - Service Dogs Selecting Using Information on Relatives Use additional information to increase accuracy of selection, such as - pedigree data - progeny data Data may be subjective or objective in nature. Information on relatives can be used when individual records are unavailable (sex-limited traits). Traits with low heritability require large amounts of information to make effective selection decisions. In livestock, some service dogs, and some horses in Europe, genetic prediction technology provides objective estimates of breeding value. - EBV - Expected Breeding Value - EPD - Expected Progeny Difference - PTA - Predicted Transmitting Ability Selection one or a few genes Simply-inherited ______________________ – affected by only a few genes. - Know genes, write out genotype. - Usually qualitative - yes/no categorical. - Phenotypes are in distinct classes. Polygenic Traits ___________________ – influenced by many genes, no single gene has an overriding influence. - Usually Quantitative, Continuous - Most of our economically important traits, but not all. - Each gene follows Mendel’s Rules. Improving simply-inherited traits - Identify animals with good alleles. Goal is the Same: Select animals with the best alleles. Improving polygenic traits - Don’t know genotypes. - Use statistical models to predict Breeding Value = Overall effect of all individuals genes for that trait. Mating Systems Between-breed selection - Make rapid change through breed differences. Mating and Mating Systems - Which Males to which Females. - System/Rules Reasons breeders use mating systems: 1. Produce offspring with very extreme breeding values. 2. Make use of complementarity use different parent to produce moderate offspring. (Corrective Mating) 3. Get Heterosis (hybrid vigor) Crossbred offspring performs better than the average of purebred parents. Fundamental difference between selection and mating Selection increases proportion of favorable genes in future generations of a population. Mating increases proportion of favorable gene combinations in future generations of a population. Mating systems and industry structure - seed stock breeders may have inbreeding. - commercial breeders usually crossbreed. Genetic improvement due to selection and mating ...
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