Recurrent selection cycle consists of three main

A recurrent selection cycle consists of three main phases:-

Individual families are created for evaluation.Parents are crossed in all possible combinations.

The plants or families are evaluated and a new set of parents selected.

The selected parents are intermated to produce the population for the next cycle of selection.

This pattern or cycle is repeated several times. The first cycle is labeled C0, and is called the base population. The subsequent cycles are named consecutively as C1, C2 . . . . . . . Cn. It is possible, in theory, to assemble all the favorable genes in a population in a single generation if plant breeders could handle a population of infinite size. However, in practice, as J.K. Frey pointed out, the technique of recurrent selection is applied to breeding with the hope that desirable genes will be gradually accumulated until there is a reasonable probability of obtaining the ultimate genotype in a finite sample.

Recurrent selection may be used to establish a broad genetic base in a breeding program. Because of multiple opportunities for intermating, the breeder may add new germplasm during the procedure when the genetic base of the population rapidly narrows after selection cycles. Research has indicated that recurrent selection is superior to classical breeding when linkage disequilibrium exists. In fact, the procedure is even more effective when epistatic interactions enhance the selective

advantage of new recombinants. Recurrent selection is applied to legumes as well as cereals.

Genetic basis of recurrent selection

Various recurrent selection schemes are available. They exploit additive, partial dominance to dominance and overdominance types of gene action. However, without the use of testers the scheme is effective for only traits of high heritability. Hence, only additive gene action is exploited in the selection for the trait. Where testers are used, selection for general combining ability and specific combining ability are applicable, permitting the exploitation of other gene effects. Recurrent for GCA is more effective than other schemes when additive gene effects are more

important. Recurrent selection for SCA is more effective than other selection schemes when overdominance gene effects are more important. Reciprocal recurrent selection is more effective than others when both additive and overdominance gene effects are more important. All three schemes are equally effective when additive with partial to complete dominance effects prevail. The expected genetic advance may be obtained by the following general formula:

where DG is the expected genetic advance per cycle, C is the measure of parental control, i is the selection intensity, VA is the additive genetic variance among the units of selection, y is the number of years per cycle, and sp is the phenotypic standard deviation among units of selection. Increasing the selection pressure will increase gain in selection provided the population advanced is not reduced to a size where genetic drift and loss of genetic variance can occur. Other ways of enhancing

genetic advance per cycle include selection for both male and female parents, maximizing available additive genetic variance, and management of environmental

variance among selection units. The formulas for various schemes are presented at the appropriate times in this textbook. The role of parental control in genetic gain can be manipulated by the breeder through the exercise of control over parents in a breeding program. When the breeder controls the genetic contributions of both parents to the selection population, the genetic gain can be twice as much as when only one of the parents is under control. Both parents may be controlled in one of several ways – selfing of selected individuals, select before pollination and recombination among selected plants only, and recombination occurs among selected clones.