The ENCU scoring system. A strategy for solving a class of single-locus genetic counseling problems.

A general rule is derived for calculating the conditional probability of a pedigree given that the consultand is a carrier. The method is designated the ENCU system. It is a generalization to accommodate all unilocal systems of inheritance with arbitrary penetrance operating independently in relatives of the ENSU system previously described (Murphy 1970a). The amount of information provided by the normal phenotypes of a person and his descendants is expressed in units of equivalent genotypically normal offspring (ENCUs). The method is applied to a typical genetic counseling problem, and it is shown that a considerable reduction in the quoted risk is obtained because the ENCU system takes into account the normal phenotypes of potential carriers of an incompletely penetrant gene.