Twin studies have played an important role in attempts to unravel the genetic contribution to diseases with adult onset, but have not been extensively utilized in efforts to establish the genetic contribution to common congenital anomalies. The paucity of twin research on congenital anomalies is partly attributable to the commonly held belief that the cause of these conditions may differ in twins and singletons. However, for many anomalies this belief is not well-substantiated. For example, it has been suggested that the etiology of nonsyndromic cleft lip with or without cleft plate (CL ยฑ P) may differ in twins and singletons, but such differences are not strongly supported by the available epidemiologic and clinical data [Christensen and Fogh-Andersen, 1993].
The present study was undertaken in an attempt to determine whether the familial recurrence pattern of CL ยฑ P differs for twins and singletons. If, in twins, CL ยฑ P occurs as the result of factors which are specific to twin conceptuses, the recurrence risks to their relatives should be lower than the risks to relatives of affected singletons. Family data for this study were obtained from two independent sources: mailed family history questionnaires (n โซืกโฌ 35) and record linkage (n โซืกโฌ 67); see Mitchell and Christensen [1996] for details regarding record linkage. Zygosity information was not available on a number of these twin pairs. Hence, all analyses are based on data from all twins combined. Data on 23 families were obtained by both family history questionnaire and record linkage. In this subset of the data, there was good agreement between the two data sources, indicating that self-reported data for