Relation of face shape to susceptibility to congenital cleft lip. A preliminary report.

It is becoming increasingly clear that the genetic basis for the predisposition to congenital cleft lip is a multifactorially determined threshold character (Fraser, 1963; Carter, 1964, 1969; Fraser, 1970). That is, a number of genetic and environmental factors, individually indistinguishable, interact to determine whether a given developmental event reaches a certain threshold necessary for normal development. If it does, the resulting baby is born without the defect; if it does not, the baby is born with a malformation. For cleft of the secondary palate the nature of the developmental event and threshold are beginning to be understood. The palate shelves must move from either side of the tongue to above the tongue and meet in the midline before growth of the head has carried them too far apart to reach each other (Fraser, Walker, and Trasler, 1957; Fraser, 1968, 1969). For cleft of the primary palate and lip (hereafter referred to as cleft lip), virtually nothing is known about the nature of the developmental threshold. Observations on the relevant susceptibility of two inbred mouse strains to cleft lip induced by maternal treatment with aspirin (Trasler, 1965) have suggested that the relevant developmental threshold may be related to the shape of the embryonic primordial face (Trasler, 1968). If the shape of the embryonic face is related to the shape of the postnatal face, and if face shape is at least in part genetically determined, and if face shape is indeed related to the predisposition to cleft lip, it follows that the parents of children with congenital cleft lip should have faces that are, on the average, of a different shape than those of the general