A diallel cross analysis of genetic determinants of copulatory behavior in rats.

A dialled cross analysis was aimed at obtaining information regarding genetic influences on the copulatory behavior of rats, and permitting considerations of adaptive significance. Rats of 4 inbred strains, ACI, F344, LEW, and WF, and all possible F1 crosses were used. Of 385 males tested, 73 failed to mate. The F1 rats were more likely to mate than were rats with inbred genotypes. Overdominance was generally apparent for low frequencies of mounts and intromissions preceding ejaculation across 5 series and 3 tests. A trend for directional dominance toward low scores on various latency measures of copulatory behavior also was apparent. According to the theory relating directional dominance to adaptation, it would appear generally adaptive for rats to copulate rapidly and to ejaculate after relatively few mounts and intromissions.