Making genetic progress in the meat goat business requires the breeder to identify genotypes of individuals for loci of interest and select those individuals with the most favorable genotypes. In selecting for polygenic traits such as weaning weight, growth rate, internal parasite tolerance, identifying specific genotypes is out of the question.

The goat breeder must therefore try to identify breeding values of individuals for traits of importance and to select those individuals with the best breeding values. This is not as straight forward as it may seem because a breeding value is an abstract, mathematical idea. It can never be measured directly, and because it is a relative concept its numerical value depends upon the breeding values of all other individuals in the population, e.g. herd or breed.

To understand breeding values we need a conceptual framework or a model in order to grasp definitions in a logical and consistent way. This framework is designed to be used with quantitative traits: traits in which phenotypes show continuous and numerical expression. The basic genetic model for quantitative traits can be written as follows:

• Phenotypic value is an individual performance record. It is the measure of a meat goat’s own performance for some specific trait.
• Genotypic value refers to the effect of the goat’s genes — singly and in combinations, on its performance for the trait. Unlike phenotypic value, it is not directly measurable.
• Environmental effect is comprised of all non-genetic factors influencing an individual’s performance for a particular trait and is not directly measurable.

Those of you who are somewhat familiar with this concept may not understand the purpose of including the population mean. The purpose for adding the mean is to emphasize that in goat breeding, genotypic values and environmental effects, and all the other elements of the framework or model are relative — relative to that population being considered. They are not absolutes. Their numerical values depend on the average performance of the population, and they are therefore expressed as deviations for the population mean. Although we might put numerical values in hypothetical situations, in reality we cannot know an individual’s genotypic value or environmental effect. All that can be measured directly is the phenotypic value.

Example 1. A young buck weighed 40.5 pounds at weaning. The population mean was 45 pounds. Therefore the phenotypic value deviation is a minus 4.5 pounds, or 10 percent below the mean…..not good. This might prevent the buck from being selected to produce offspring, which would be appropriate if he is genetically inferior. It is conceivable however that the genotypic value of this buck is 2.5 pounds above average, but his actual performance is below average due to a very poor environment which put a 16 percent drag on the buck. Perhaps his mother developed mastitis in one half of her udder. In that case we might have overlooked a genetically superior sire because of poor animal husbandry.

Example 2. Another buck was in a population in which mean postweaning average daily gain equals .50 pounds per day. This guy recorded an ADG of .45 lb per day. The environmental effect accounted for .04 lbs of this negative deviation and his genotypic value accounted for .01 pounds. In the end this buck should not be selected for breeding because although he experienced worse than average environmental effect, he also had a lower than average genotypic value for ADG.

Example 3. Our final buck weighed 100 pounds at seven months of age which was about 20 percent above the population mean of 83 pounds. Obviously this buck should be considered as herd sire material and be put in the “for sale pen” of higher priced bucks. What we do not know at this point is that the genotypic value of this buck is only 6 percent above the population mean, and that a positive environmental effect added an extra 14 percent to allow him to reach the 20 percent superiority. Although he has a higher than average genotypic value, the rate of growth of his offspring may be less than expected.

The breeding value of an individual is contained in the genotypic value, which includes independent gene effects (additive effects or breeding value) which are passed along to the next generation, and gene combination effects which are not passed because the law of recombination breaks up the combinations on the way to the next generation.

One can think of genotypic value as the value of an individual’s genes to its own performance, and breeding value as the value of an individual’s genes to its progeny’s (offspring) performance.

Fundamental genetics for understanding what is happening in cells

During the late 19th and early 20th century, the Austrian monk Gregor Mendel and later on several of his successors identified the fundamental mechanisms and rules of inheritance. This knowledge allows you to better comprehend the important concepts of goat breeding. These concepts apply generally to meat goats like they do to other livestock. There is no evidence that goats are an exception to any of the basic biological principals. Appendix B provides the reader with detailed information on these basic concepts, and acquaint the learner with the useful jargon of genetics.

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