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notes - Notes ANSC 4018 Chapter 7 The Genetic Model for...

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Notes ANSC 4018 Chapter 7: The Genetic Model for Quantitative Traits 1. In selecting for simply inherited traits, the breeder’s task is to identify genotypes of individuals for loci of interest and select those individuals with the most favorable genotypes. 2. The breeder’s job is to identify breeding values of individuals for traits of importance and to select those individuals with the best breeding values. 3. Traits can never be measured directly and, being a relative concept, its numerical value depends upon the breeding values of all the other individuals in the population. 4. The genetic for quantitative traits is designed to be used with quantitative traits: traits in which phenotypes show continuous (numerical) expression. However, in practice, the model is used with polygenic traits (qualitative). Qualitative traits are threshold traits and are not continuously expressed, at first glance, and do not appear to be good candidates for a quantitative model. 5. Quantitative Genetics: the branch of genetics dealing with the genetic model for quantitative traits and its applications. a. It is concerned with influences on, measurement of, relationships among, genetic prediction of, and rate of change in traits that are or can be treated as quantitative. 6. The basic genetic model for quantitative traits is represented by the following equation: P = μ + G + E Where: P = the phenotypic value or performance of the individual animal μ = the population mean or average phenotypic value for the trait for all animals in the population G = the genotypic value of the individual for the trait E = the environmental effect on the individual’s performance for the trait 7. A phenotypic value is an individual performance record; it is the measure of an animal’s own performance for a specific trait. 8. Genotypic value refers to the effect of the individual’s genes (singly and in combination) on its performance for the trait; not directly measured. 9. The environmental effect is comprised of all non-genetic factors influencing an individual performance for a trait. 10. The numeric value of μ depends on the average performance of the population, and they are therefore expressed as deviations from the population mean. 11. Value is any measure applied to an individual as opposed to a population. a. Ex: phenotypic values, genotypic values, breeding values, and environmental effects. 12. Because G and E are expressed as deviations from a mean, the average of genotypic values and the average of environmental effects across an entire population are zero. 13. The genotype of the individual has no influence on the environmental effect that the individual experiences and vice versa. 14. In selecting for polygenic traits, breeders try to choose those parents of individuals with the best sets of genes (best breeding values).
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