| The development of breeding populations from two related inbreds restricts the amount of variability expected in the cross. The first objective of this research was to determine if, as expected from an additive genetic model, genetic variance in maize (Zea mays L.) decreases as the number of parents of the population decreases. Eight F3 families, derived from the same single cross, were intermated in a hierarchical manner to form populations with N = 1, 2, 4, or 8 parents. Testcross genetic variance (VTC) generally decreased with N, although differences observed at N = 2, 4, and 8 were mostly insignificant. In particular, grain yield VTC at N = 4 was not significantly different from the VTC in the base population, indicating a genetic variance in excess of what is predicted by an additive genetic model. The results implied that non-additive gene effects in elite maize inbreds help maintain genetic variance in small populations. Consequently, the second objective of this research was to determine how multi-locus epistasis and linkage affect the loss of genetic variation in populations established from small N. In simulation experiments, the average ratio between genetic variance in the progeny population (VN) and the base population (VB) was lower under metabolic flux epistasis than under additivity, indicating that epistasis hastens the decline in genetic variance due to small N. In contrast, the 95th (Delta95%) percentile difference between the estimated VN/VB and the expected VN/V B was higher with epistasis than with additivity across the different levels of N and L. Linkage had little effect on VN/VB, whereas it increased Delta95% under both additivity and epistasis. While metabolic flux epistasis led to a faster average decline in genetic variance, it also led to greater variability resulting in some populations having VN/VB that was larger than expected. |
