From complex traits to complex alleles

Several recent studies using natural populations of Drosophila show that on e must be very careful when sorting among the targe number of molecular pol ymorphisms found at most loci to identify the nucleotide changes responsibl e far phenotypic variation in complex traits. Indeed, several mutations wit hin a single allele can interact to generate the overall observed effect. T he results are instructive both for those interested in the genetics of evo lutionary change and for those attempting to ferret out the genetic basis o f complex human diseases.