Heritability of liability and concordance in monozygous twins

SUMMARY Results from heritability of liability model of Falconer (1965) have been revised, eliminating two sources of bias. The results have been graphed for convenient usage, and in the absence of environmental similarities among relatives can apply to all kind of relatives. Confidence levels for the heritability (or correlation) estimates can also be read directly off the graph. The results also apply to concordance rates in MZ twins. They show that, in the absence of environmental similarities, concordance rates for a disease in MZ twins will only be high if the heritability is very high. Thus a low concordance rate in MZ twins cannot be taken to prove that genetic factors are not important in the predisposition to a disease.