Genetic penetrance from MZ twins of absent C-triradius, a palmar dermatoglyphic trait.

Among 59 sets of MZ male twins and 48 sets of MZ female twins from Italy, 7 male sets (11.9%) and 7 female sets (14.6%) had absent c-triradius on one or more palms; among 214 individuals who had a MZ twin, 19(8.9%) had absent c-triradius in one or both palms. Due to small numbers these percentages are comparable to the occurrence of absent c-triradius found in 8.2% of 3946 Caucasians. When one member of a set of MZ twins showed the trait in one or both palms, 35.7% (5/14) of the other members also showed the trait in one or both palms. When an individual showed the trait in one palm, 26.3% (5/19) of the other palms also showed the trait. These two estimates of penetrance combined give an average value of 30.3% (10/33) which is comparable to an estimate of 30% found in a study of 478 family units in which transmission was compatible with an autosomal dominant gene.