THE analysis of the effects of single genes on behaviour may yield information not only about the genetics of behavioral characters, but about the characters themselves1. When, however, data are collected on segregating populations for a few generations apparent single gene effects may be due to the effects of closely linked loci, especially when these populations are derived from an initial cross of parental strains which differ widely in the character in question. Thus when apparent major-gene effects are found, it is important to verify that such effects are truly pleiotropic. © 1969 Nature Publishing Group.