INTRODUCTION Genetic variation within a species can lead to phenotypic variation of two types, continuous (quantitative) and discontinuous (qualitative). Discontinuous variation permits the unambiguous assignment of individuals to alternative classes, even though there may be continuous variation within such classes. Frequently the different phenotypic classes are caused by differences at a single genetic locus, as in phenylketonuria or in haemophilia. When the observed variation is continuous genotypes can only be assigned unambiguously to individuals which have been progeny-tested. If the contribution of non-genetic (environmental) factors to the observed phenotypic variability is large (more than 50%) the variation may appear continuous, even though only one or two major genetic loci are involved. Even when environmental factors make a small contribution, variation in the observed character may be continuous if there are a large number of genes ('polygenes') involved. An apparently discontinuous distribution may result from a fundamentally continuous one if there