Selection Methods for Genetics Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Family and Pedigree Selection

Family Selection

When both broad and narrow heritabilities of a trait are low, environmental variance is high compared with genetic variance. Family selection (also referred to as kin selection) is most useful when heritabilities of traits are low and family members resemble one another only because of their genetic relationship. It is usually more practical to first reduce environmental variance before initiating selective breeding programs. Another way to minimize the effects of an inflated environmental variance is to save for breeding purposes all members of families that have the highest average performance, even though some members of such families have relatively poor phenotypes. In practice, it is not uncommon to jointly use more than one selection method; e.g., choosing only the top 50% of individuals in only the families with the highest averages.

Family selection is most beneficial when members of a family have a high average genetic relationship to one another but the observed resemblance is low. If inbreeding increases the average genetic relationship within a family more than the increases in phenotypic resemblance, the gain from giving at least some weight to family averages may become relatively large.

Pedigree Selection

In this method, consideration is given to the merits of ancestors. Rarely should pedigree selection be given as much weight as the individual's own merit unless the selected traits have low inheritabilities and the merits of the parents and grandparents are much better known than those of the individual in question. It may be useful for characteristics that can only be seen in the opposite sex or for traits that will not be manifested until later in life, perhaps even after slaughter or harvest. The value of pedigree selection depends upon how closely related the ancestor is to the individual in the pedigree, upon how many ancestors' or colateral ancestors' records exist, upon how completely the merits of such ancestors are known, and upon the degree of heritability of the selected traits.

Progeny Test

A progeny test is a method of estimating the breeding value of an animal by the performance or phenotype of its offspring. It has its greatest utility for those traits that (1) can be expressed only in one sex (e.g., estimating the genes for milk production possessed by a bull), (2) cannot be measured until after slaughter (e.g., carcass characteristics), or (3) have low heritabilities so that individual selection is apt to be highly inaccurate.

Progeny testing cannot be practiced until after the animal reaches sexual maturity. In order to progeny-test a male, he must be mated to several females. If the sex ratio is 1 : 1, then obviously every male in a flock or herd cannot be tested. Therefore, males that have been saved for a progeny test have already been selected by some other criteria earlier in life. The more progeny each male is allowed to produce the more accurate the estimate of his "transmitting ability" (breeding value), but, in so doing, fewer males can be progeny-tested. If more animals could be tested, the breeder would be able to save only the very best for widespread use in the herd or flock. Thus, a compromise must be made, in that the breeder fails to test as many animals as desired because of the increased accuracy that can be gained by allotting more females to each male under test.

The information from a progeny test can be used in the calculation of the "equal-parent index" (sometimes referred to as the "midparent index"). If the progeny receive a sample half of each of their parents' genotypes and the plus and minus effects of Mendelian errors and errors of appraisal tend to cancel each other in averages of the progeny and dams, then average of progeny = sire/2 + (average of dams)/2 or

Sire = 2(average of progeny)–(average of dams)

Practice problems for these concepts can be found at:

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