Discontinuity and quasi-continuity: Alternative hypotheses of multifactorial inheritance

Consequences of three models of multifactorial inheritance are deduced for inbreeding effects and recurrence risks, and methods are developed to test goodness of fit. Applying the computer program DISQUAC to several bodies of data, we find that every one which fits a quasi-continuous modcl also fits a discontinuous one. Our results show that it is exceedingly difficult, and may be practically impossible, to infer the genetic basis of traits which do not give regular mendelian ratios. The only consolation is that predicted recurrence risks are esscntially the same for similar models. Thus the two quasi-continuous models of Falconer and Edwards, which assume cumulative gene action, seem in practice indistinguishable, while the genetic load model can be differentiated only if there is significant dominancc. Useful predictions for genetic counseling may be made from a well-fitting model even when the mode of inheritance is unclear. In particular, it is shown how to estimate the probability of affection in the next child after s sibs have been born, of whom r were affected, when there is no birth order effect.