The nature of genetic influences on behavior: Lessons from "simpler" organisms

Substantial advances have been made in recent years in the understanding of the genetic basis of behavior in "simpler" organisms, especially the mouse and the fruit fly Drosophila. The authors examine the degree of similarity between the genetic underpinnings of psychiatric illness and genetic influences on behavior in such simpler organisms. Six topics are reviewed: 1) the extent of natural genetic variation, 2) the multigenic nature of natural variation, 3) the impact of individual genes on multiple traits, 4) gene-environment interactions, 5) genetic effects on the environment, and 6) gene-by-sex interactions. The results suggest that the pattern of results emerging in psychiatric genetics is generally consistent with the findings of behavioral genetics in simpler organisms. Across the animal kingdom, individual differences in behavior are nearly always influenced by genetic factors which, in turn, result from a substantial number of individual genes, each with a small effect. Nearly all genes that affect behavior influence multiple phenotypes. The impact of individual genes can be substantially modified by other genes and/or by environmental experiences. Many animals alter their environment, and the nature of that alteration is influenced by genes. For some behaviors, the pathway from genes to behavior differs meaningfully in males and females. With respect to the broad patterns of genetic influences on behavior, Homo sapiens appears to be typical of other animal species.