Sex and U

Resolution of several unsettled problems in genetics depends on the genomic rate of deleterious mutation, U.Selection against mutations can be a major factor in evolution only if U greater than or equal to1. Recently, signifi cant progress has been made in measuring U in multicellular eukaryotes. An indirect estimate, based on a human-chimpanzee pseudogene comparison, produ ced U>3 for hominoids. By contrast, an estimate for Drosophila based on com parison of synonymous protein-coding sites produced U<0.1. However, the Dro sophila figure might be underestimated because of selection at synonymous s ites. Perhaps,the best way to measure U is to observe mutations shortly aft er they appear. So far, this direct approach has been applied only to human s and Caenorhabditis elegans, yielding high estimates of mutation rates.