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Reduction in the frequency of' undesirable recessive genes is examined for several combinations of initial gene frequencies and number of offspring obtained by random mating of young sires to the population. Only sires not having affected offspring are then returned to heavy use. Such selection can very rapidly reduce the number of affected offspring, if the initial gene frequency is high or if the number of tested offspring is 200-500 for lower gene frequencies. For some combinations, frequency of affected offspring in the population is higher after two or three generations of selection than in the first generation of selection. With high initial gene frequencies, practically all heterozygous males are detected and culled during the test period, resulting in practically no affected offspring in the first generation. The population gene frequency remains relatively high, due to the influence of the unselected females. In the next generation the heterozygous males are not detected with high probability and the frequency of affected offspring increases above that of the first generation. After the new peak of affected offspring is reached in a few generations, frequency drops, as expected.