On the effectiveness of early generation selection in self-pollinated crops

Breeders of self-pollinated crops often discard inferior lines at an early selfing generation so that more resources can be devoted to the further testing and selection of the more promising lines. Empirical studies have led to contradictory estimates of the correlation between the performance of lines at early and late selfing generations. Here I examine the theoretical effectiveness of early generation selection. When dominance is absent, the genetic correlation (r_G) between the performance of an F_t-derived F_g line (i.e., F_t:g) and a descendant homozygous line is equal to the square root of [1 + F_(t)]/2, where F_(t) is the inbreeding coefficient at generation t. Dominance, when present, has little effect on r_G. The minimum value of r_G is high; that is, 0.707 for an F₂-derived line. From a genetic standpoint, early generation selection is expected to be effective, but in practice it becomes ineffective if nongenetic effects are large.