The study of twins is a well-established method for evaluating the relative roles of heredity and environmental factors in the etiology of diseases. Conclusions depend on zygosity determination and on the classification of minor forms of diseases. This paper reports on ten (5 mono- and 5 dizygotic) out of thirteen twin pairs among 1039 patients with cleft lip and palate (n = 677) or cleft palate (n = 362). Zygosity was determined using “DNA fingerprinting” on blood samples In all 10 pairs and on cleft-associated tissue in one pair. Including minor forms of clefting, two of five pairs of monozygotic and two of five pairs of dizygotic twins of the same sex showed concordance. “DNA fingerprinting” should be established as a definitive method for zygosity determination, and the calculation of concordance rates should always include minor forms of diseases.
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