Incidence of cleft lip and palate in the offspring of cleft parents.

The results of this study support the idea that CL(P) is a polygenic trait. Furthermore, employing the calculations of Edwards, there was little evidence for major gene effects. The sex-modified threshold concept which has been made an integral part of the CL(P) polygenic model was not entirely confirmed by these data. These data show an affected offspring risk for CP parents of both sexes of about 6 %. Although Edward's calculations failed to provide evidence for major gene effects in CP, there was an unexplained five-fold difference between CP frequency in offspring and sibs which needs further study.