THREE methods are available for the genetic analysis of continuous variables in human populations. One is the biometrical approach1 which, in addition to partitioning the phenotypic variance into genetic and environmental components, has been used to reveal sex linkage2 and provide estimates of the number of effective factors3. Second, genetic markers can be studied for evidence of pleiotropic effects4, and third, linkages between polygenic factors and segregating genetic markers can be sought5,6. It is not always possible to distinguish between linkage and pleiotropy, so the second and third methods need similar data and thus go together.