INDEX MEASURES FOR ASSESSING THE MODE OF INHERITANCE OF CONTINUOUSLY DISTRIBUTED TRAITS .1. THEORY AND JUSTIFICATIONS

A class of indices that may be applied to quantitative data on nuclear families and that can help to assess degrees of mode of inheritance is developed. Given phenotype values of spouses x(1) and x(2) and offspring y, the deviation of an offspring value from the midparent is |y - 1 2(x(1) + x(2))|, and those from the separate parents are |y - x(1)| and |y - x(2)|. The indices called major-gene indices (MGI) investigated are functions of the deviations from midparental values compared to corresponding symmetric functions of the deviations from separate parents. Major-gene indices exceeding 1 may indicate some extent of major-gene inheritance, whereas an MGI less than 1 is suggestive of relatively more polygenic inheritance. Superposition of assortative mating and environmental effects will tend not to shift the MGI greater than 1 for polygenic inheritance, nor will they shift the MGI less than 1 for major-gene factors. The reliance on the proposed indices is reinforced on the basis of a hierarchy of representative models of monogenic and multifactorial inheritance. Extensions of the method to deal with multigenerational pedigrees are briefly discussed. © 1979.