Evolutionary geneticists measure genetic variation in phenotypic traits by analyzing the correlations among relatives and expressing the results on a scale that runs from zero to one. Those values are called the heritabilities of the traits; the symbol for heritability is h2. Heritability has a technical meaning that differs from its meaning in everyday language. A heritability of 1.0 means that all offspring have exactly the same value of a trait as the average of the two parents. A heritability of 0.0 means that relatives do not resemble each other any more than would two unrelated people chosen at random. Heritability measures the proportion of total phenotypic variation in the population that can be accounted for by the effects of many genes acting additively, unconstrained by interactions with other genes. Other than genetic effects, the major source of trait variation among individuals is the environment in which they have been raised.