Facial clefts in Danish twins.

A total of 74 Danish twin pairs with cleft lip +/- palate (CL(P)) and isolated cleft palate (CP) born in Denmark from 1941 to 1969 were studied. Eight pairs were of indeterminate zygosity status' and 26 pairs of unlike sex were dizygous (DZ). Of the remaining 42 like-sexed pairs, zygosity assignments were made from genotyping and physical resemblance data. Twelve pairs were given MZ status and thirty pairs DZ status. The following data was calculated: 1) in contrast to other reports, the incidence of either CL(P) or CP was not increased for either MZ or DZ twins; 2) using the pairwise method for concordance rate calculation, concordance rates for CL(P) twins were: MZ = 36 per cent; DZ = 1.5 per cent. For CP, MZ = 33 per cent; DZ = 0 per cent. The results support the concept that heredity is a prime factor in the etiology of clefting, but the low MZ concordance rates also suggest genetic heterogeneity in this cleft population.