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Unformatted text preview: together with h 2 can be of use to the breeder. Suppose that h 2 and H 2 are both low, which means that there is a large proportion of environmental variance compared with genetic variance. Some scheme of reducing must be used. One method is to change the husbandry condi- tions so that environmental variance is lowered. Another is to use family selection. Rather than selecting the best individuals, the breeder allows pairs to produce several trial progeny, and parental pairs are selected to produce the next generation on the basis of the average perfor- mance of those progeny. Averaging over progeny allows uncontrolled environmental variation and developmen- tal noise to be canceled out, and a better estimate of the genotypic difference between pairs can be made so that the best pairs can be chosen as parents of the next generation. If, on the other hand, h 2 is low but H 2 is high, then there is not much environmental variance. The low h 2 is the result of a small proportion of additive genetic vari- ance compared with dominance variance. Such a situa- tion calls for special breeding schemes that make use of nonadditive variance. One such scheme is the hybridinbred method, which is used almost univer- sally for corn. A large number of inbred lines are cre- ated by selfing. These inbred lines are then crossed in many different combinations (all possible combinations, if it is economically feasible), and the cross that gives...
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- Spring '08