Recurrent Selection for Yield in the F2 of a Maize Single Cross 1

Recurrent selection for intracross progeny yield in the F 2 of a maize ( Zea mays L.) single cross was continued through four cycles to measure population changes and to interpret the genetics of those changes. Because there are no multiple alleles in single crosses and gene frequencies are 0.5 at all heterozygous loci, gene shifts might be more clearly interpreted in such a population than in heterogeneous ones. Pairs of F 3 . plants were crossed and yield‐tested; the population was advanced by diallel crossing the most productive crosses each cycle. Tests were conducted of 208 CO, 190 C1, 105 C2, 62 C3, and 101 C4 crosses. Over the four cycles, increases in percent of F 1 yield were from 59.9 to 80.4 for mean yield, from 91.6 to 99.8 for maximum progeny yield, and from 31.6 to 58.8 for minimum progeny yield. In both C8 and C4 the lowest progeny yield exceeded that of the F 2 check, although not significantly. It seems apparent from the data that the overall frequency of desirable genes increased, but some deleterious genes must have been maintained or increased in frequency sufficiently to suppress the maximum potential among the genotypes. The cumu. lative depressive effect of homozygous deleterious genes on homozygous dominant or heterozygous loci in the F 2 was estimated to be 26.7% or greater, assuming complete dominance. A negative correlation was found between date of F 2 plant pollination and yield of their progeny, suggesting that the earlier segregates may have carried a lighter load of deleterious genes. If so, selection of earlier segregates may be an additional useful breeding tool.