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An increased twinning rate has been observed in the near relatives (sibs, parents, and aunts and uncles) of probands with neural tube defects (NTDs) occurring at the level of the 11th thoracic vertebra and above (upper NTDs). The twin rate was more than double that of the near relatives of probands with lower NTDs and of those of probands with Mendelian disorders (the controls). The excess twinning was same sex and can therefore consist of either MZ or same-sex DZ twins. Furthermore, upper-NTD families with twins had a higher NTD-sibling occurrence rate than did families without twins. These findings, if corroborated, may imply an etiology common to twinning and NTDs and can perhaps be applied in counseling NTD families.

Effects of parental consanguinity on morbidity and mortality can be estimated from observations on families ascertained through a child with a disease or defect, provided that appropriate corrections are made for the ascertainment bias. The risk of first-cousin parents having a child with a recessively inherited disease appears to be low (less than 1%). Data obtained by this approach suggest that the increased infant mortality associated with inbreeding (upon which calculations of lethal equivalents are based) may result in part from environmental differences between consanguineous and nonconsanguineous matings, which may be changing. Thus estimates of the number of lethal equivalents in a population can change as the environment changes.