Selection Methods for Genetics Help
Artificial vs. Natural Selection
Artificial selection is operative when humans determine which individuals will be allowed to leave offspring (and/or the number of such offspring). Likewise, natural selection allows only those individuals to reproduce that possess traits adaptive to the environments in which they live. There are several methods by which artificial selection can be practiced.
If heritability of a trait is high, most of the phenotypic variability is due to genetic variation. Thus, a breeder should be able to make good progress by selecting from the masses those that excel phenotypically because the offspring-parent correlation should be high. This is called mass selection, but it is actually based on the individual's own performance record or phenotype. As the heritability of a trait declines, so does the prospect of making progress in improving the genetic quality of the selected line. In practice, selection is seldom made on the basis of one characteristic alone. Breeders usually desire to practice selection on several criteria simultaneously. However, the more traits selected for, the less selection "pressure" can be exerted on each trait. Selection should thus be limited to the two or three traits that the breeder considers to be the most important economically. It is probable that individuals scoring high in trait A will be mediocre or even poor in trait B (unless the two traits have a positive genetic correlation, i.e., some of the genes increasing trait A are also contributing positively to trait B). The breeder therefore must make compromises, selecting on a "total merit" basis some individuals that would probably not be saved for breeding if selection was being practiced on the basis of only a single trait.
The model used to illustrate the concept of genetic gain [Fig. 8-9(a)], wherein only individuals that score above a certain minimum value for a single trait would be saved for breeding, must now be modified to represent the more probable situation in which selection is based on the total merit of two or more traits [Fig. 8-9(b)].
In selecting breeding animals on a "total merit" basis, it is desirable to reduce the records of performance on the important traits to a single score called the selection index. The index number has no meaning by itself, but is valuable in comparing several individuals on a relative basis. The methods used in constructing an index may be quite diverse, but they usually take into consideration the heritability and the relative economic importance of each trait in addition to the genetic and phenotypic correlations between the traits.
An index (I) for three traits may have the general form
where a, b, and c are coefficients correcting for the relative heritability and the relative economic importance for traits A, B, and C, respectively, and where A', B', and C' are the numerical values of traits A, B, and C expressed in "standardized form." A standardized variable (X') is computed in a sample by the formula
where X is the record of performance made by an individual, is the average performance of the population, and s is the standard deviation of the trait. In comparing different traits, one is confronted by the fact that the mean and the variability of each trait is different and often the traits are not even expressed in the same units.
EXAMPLE 8.15 An index for poultry might use egg production (expressed in numbers of eggs per laying season), egg quality (expressed in terms of grades such as AA, A, B, etc.), and egg size (expressed in ounces per dozen).
EXAMPLE 8.16 An index for swine might consider backfat thickness (in inches), feed conversion (pounds of feed per pound of gain), and conformation score (expressing the appearance of the individual in terms of points from a standard grading system).
The standardized variable, however, is a pure number (i.e., independent of the units used) based on the mean and standard deviation. Therefore, any production record or score of a quantitative nature can be added to the score of any other such trait if they are expressed in standardized form.
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