Neural tube malformations: complex segregation analysis and recurrence risk.

A sample of 223 families with at least one child with anencephaly and/or spina bifida was ascertained in Southern Poland, where the incidence of neural tube malformations is 0.92/1,000. The recurrence risk in proband's sibs was 3.4%. The calculated heritability (h2) was 76%, implying a substantial contribution of genetic factors to the cause of neural tube defects. Complex segregation analysis was applied in an attempt to discriminate between the hypothesis of a single locus and that of quasi-continuity under multifactorial inheritance. The results excluded the hypothesis of dominant inheritance with full penetrance. The hypothesis of the two-allele model (at a single locus) fit the data with the same degree of exactness; however, for all hypotheses the estimated penetrance was low, and the phenocopy frequency was high. Less accurate conformity was observed for the multifactorial hypothesis. In view of very low penetrance and high frequency of phenocopies in single locus hypotheses and the finding that empirical data gave better support to the multifactorial model, the differences in chi 2 values should not be viewed as sufficient to discriminate between single locus and multifactorial models. The recurrence risks, assessed in the complex segregation analysis, showed an increase together with the growing number (r) of affected children. For constant r the recurrence risks decreased in successive pregnancies. Although discrimination between the mendelian and multifactorial inheritance models was incomplete, the risk values obtained can be employed in genetic counseling.