SUMMARY A large body of data on segregating families is used to generate specific recurrence risks conditional on sex and birth order for the best-fitting model of polygenes plus maternal effect. The method is general for diseases of complex inheritance, and lies within the competence of any serious genetic clinic. The question of whether consultees demand as much specificity should be subordinate to the question of whether counsellors are justified in providing less.Genetic counselling is a transaction which includes specification of a recurrence risk for a particular disease in a particular family, using parameters from segregation analysis of a larger body of data. This problem has been solved for simple cases (Murphy and Mutalik, 1969), and the utility of computer programmes to determine specific risks has been *demonstrated (Heuch and Li, 1972). It remains to generalise these prototypes to more complex pedi-*grees and modes ofinheritance.As a first step we here apply the mixed model of Morton and MacLean (1974) to a large body of data on pyloric stenosis, a disease of obscure and presumably complex aetiology whose incidence varies widely with sex, birth order, and ethnic group. The mixed model subsumes polygenes, major loci, and common environment. Application of the mixed model illustrates the power and specificity which complex segregation analysis brings to prediction of recurrence risks.
Pyloric stenosisDiagnosis of a hypertrophic muscular tumour of the pylorus presents several problems. The tumour may be present without causing symptoms at any time (Carter and Powell, 1954). Distinction from pylorospasm is not clear.