Infinitesimal Model

Traditional selection methods have assumed that performance traits are controlled by an infinite number of gene loci, each with an equal-sized contribution to the trait of interest. Only purely additive gene action has been assumed to exist, i.e., the genetic effects that are passed to progeny. Dominance effects and other interactions between loci have been assumed to be negligible.

can be much higher. Intensity of selection is a function of reproductive output compared to the number of males and females for creating the next generation. The generation intervals for males and females could be different depending on when selection decisions are made for each sex. In dairy cattle, for example, sires are not culled until their first daughters have matured, calved, and completed a full lactation of milking. Thus, sires can be six years of age or older. Cows are often culled based on their own performance, somewhere between 20 and 36 months of age.

Response to selection can be increased in several ways. Accuracy of evaluation can be increased by using more data and better methods of evaluation. Intensities of selection can be increased by selecting fewer animals from among the possible candidates. Generation intervals can be shortened by making selection decisions sooner in an animal's life. All of these factors must be balanced against the costs of achieving them.

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