In clasping the hands, some persons place their right thumbs on their left ones (R-situation) and the others have the opposite habit (L-situation). Statistics of family studies by Lutz (1908), Yamaura (1940), Kawabe (1949, 1953) and Freire-Maia et al (1958) have confirmed that this trait is hereditary. The author has analyzed these data by a mathematical method of population genetics established by Goto (1954, 1957) to determine the mode of inheritance, the gene frequency and the manifestation rate. The trait of R-situation is inherited by one dominant gene and not only all of homozygous dominants and heterozygotes but also a part of homozygous recessives (the genotype of L-situation) manifest the R-situation by a phenocopy. The frequency of this gene has a racial difference, which is higher in Japanese than in Europeans. The manifestation rate i.e. the rate of phenocopy is higher in adults than in children.
