Analysis of the diallel mating design for maize inbred lines

One of the assumptions required to estimate genetic parameters by use of the diallel mating design is that the genes in the parents are independently distributed. The objective of this study was to test the validity of this assumption. Diallel experiments including a fixed sample and a random sample of parents were conducted in maize (Zea mays L.). For the fixed sample, an eight-parent diallel among selected inbred lines was produced (original diallel). Ninety-six unselected inbred lines derived from a population, developed by intermating for five generations crosses of the original diallel, were used to produce 12 eight-parent diallels (random diallels). Diallels were evaluated in a replication-within-sets randomized incomplete block design in six environments. Estimates of additive (sigma(A)(2)) and dominance (sigma(D)(2)) variances from the pooled random diallels were significantly different from estimates of sigma(A)(2) and sigma(D)(2) in the original diallel for about half the traits. For six traits, the average level of dominance was overestimated in the original diallel relative to the pooled random diallels. The estimated average level of dominance for grain yield was two times greater in the original diallel than in the pooled random diallels. Estimates of sigma(A)(2) seemed to be affected more than estimates of sigma(D)(2) by repulsion phase linkages. Non-independent distribution of genes causes differences in sigma(A)(2) and sigma(D)(2). The diallel mating design should only be used to estimate genetic parameters when the parents of the diallel have been randomly selected from a population in linkage equilibrium.