319 Selection

319 Selection - SELECTION SELECTION: a process by which...

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SELECTION SELECTION: a process by which certain individuals in a population have an ADVANTAGE in the production of the next generation. Natural Selection : survival of the fittest. Even in unmanaged populations, certain individuals are going to have an advantage in a specific environment. The advantage may be WITHIN a species or BETWEEN species. Example/within: In the tropics, a bull that has sweat glands can be more active during the heat of the day. Therefore, he has more opportunities to breed cows. Example/between: Wolves in the wild kill the weaker, slower mountain sheep. The dead sheep do not produce the next generation. In most cases of natural selection, the ENVIRONMENT provides the selection pressure. In a livestock production situation, environment remains a definite factor in selection. Early death, injury, drought, etc., are all NATURAL conditions that influence the selection process. In the wild, unmanaged state, there is still a tendency toward improvement of the population through selection. Inferior, weak, genotypes due to defective or detrimental genes or gene combinations are eliminated. New genes (mutations) and new gene combinations that provide an advantage in a specific environment are favored, and the frequency of these genes increases within the population. Natural selection favors those traits the increase the SURVIVABILITY of the population. These traits may, or may not be of economic importance. However, the animal breeder should never neglect the impact of natural selection.
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selection practiced by MAN. The efforts of man to increase the frequency of desirable genes, or combinations of genes, in a population by locating and saving for breeding purposes those individuals with superior performance, or those individuals that produce superior performing offspring. Selection does NOT create new genes. Selection is practiced to INCREASE the frequency of desirable genes; To DECREASE the frequency of undesirable genes; and/or To influence the COMBINATION of genes that influence a specific trait (adaptability to environment). SELECTION FOR NON-ADDITIVE GENE ACTION: 1. Dominance/recessiveness A- = desirable; aa = undesireable Culling all undesirable animals (aa) increase frequency of A *Problem = no difference between AA and Aa. SUCCESS depends upon the number of heterozygous (Aa) in the population. A- = undesirable; aa = desirable Culling all undesirable animals frequency of aa = 100%. 2. Co-dominance/over-dominance AA<Aa>aa; Aa = most desirable. Cull all Aa individuals!!! WHY? 3. Epistasis interaction between genes which are not alleles. Certain combinations of gene pairs produce a superior individual when compared to other individuals with different combinations. The average of the offspring exceeds that of the parents =
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319 Selection - SELECTION SELECTION: a process by which...

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