Selection in a population with overlapping generations

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Abstract
SUMMARY A theoretical investigation is reported into the effects of overlapping generations on the prediction of genetic gain from repeated within-population truncation selection for a quantitative trait. The gain resulting from the selection process produces genetic differences between the groups of breeding animals of different ages, and between the means of their progeny. These differences are not allowed for in the usual prediction formula. The correct allowances to make for the genetic effects of maternal age are derived for a simple situation. The implications of these allowances on the overall rate of genetic progress, and on the optimum age structure for maximum progress, are shown in a model pig breeding programme. The normal prediction formula underestimates the genetic gain which can be achieved if females are kept for five litters by 10 to 12%, and overestimates the disadvantages of keeping females for more than one or two litters.