The genetic basis of cleft lip and palate deformity.

The embryology of the normal lip and palate and the abnormal development of these structures causing cleft lip (CL) and cleft palate (CP) are discussed. As the development of the lip and palate is not dependent on a single factor but on many factors, it is understandable that CL and CP may occur as a result of abnormalities of genes of large mutant effect (i.e. Mendelian mutant genes), or due to chromosomal anomalies. However, these 2 causes are relatively uncommon and the commonest mode of production is dependent on both genetic and environmental factors. The mechanisms whereby these 3 groups may produce CL and CP will be discussed. Since the modes of inheritance differ in the 3 groups, it is important to discover which group a particular patient belongs to. The genetic load is high in the Mendelian group, is low usually in those due to chromosome disorders, and may be low or high in the last group depending on the polygenes as well as environmental factors. In this last group, empiric risk figures may be calculated for future offspring to be affected. The prenatal diagnosis of CL and CP is also discussed.