Each of two sorghum (Sorghum bicolor (L.) Moench) cultivars were crossed with representatives of three wild sorghum races. Backcross-derived sorghum populations containing 3.125 to 50% wild germplasm were evaluated for grain yield, 100-kernel weight, days to flower, and plant height. Population means increased linearly with backcrossing for kernel weight, increased curvilinearly for grain yield, decreased curvilinearly for plant height, and changed erratically for days to flower. For all traits, the relationship between genetic variance and level of backcrossing deviated significantly from that expected based on an additive model. Genetic variance usually reached a maximum in the BC1 or BC2. The BC1 genetic variance for grain yield, averaged over matings, was twice as large as the average BC0 genetic variance. An epistatic model involving gene regulation is proposed as a plausible explanation for the results.