Arguments against intermating before selection in a self-fertilising species.

The expected benefit from intermating the F 2 population from a cross of two homozygous parents is investigated. It is noted that intermating will only be of benefit for two loci if the alleles are initially in ths repulsion phase, and this result is extended to more than two loci by population simulation. If V n is the population variance after n cycles of intermating followed by repeated selfing, the merit of intermating is measured by the magnitude of V 1/V0. For a character controlled by loci on a short chromosome segment the ratio is invariably greater than 1.0, but as the average recombination value between loci is increased, either by increasing the chromosome length or by situating the loci on different chromosomes, individual values less than 1.0 become possible. If loci are spread over three or more chromosomes then the expected gain from intermating is consistently small. A simulation study shows that truncation selection is usually preferable to intermating as a procedure for increasing the proportion of desirable homozygotes in a population.