第III型双生児の存否に関する問題

Abstract

It is generally recognized that human twins are of two types, monozygotic (MZ) and dizygotic (DZ), the former being so much alike that they can be hardly distinguished from one another ; while the latter are no more alike than ordinary brothers and sisters. There is usually no difficulty in distinguishing MZ from DZ twins, but sometimes cases arise in which the distinction is by no means clear and these have led to the possibility of a third type of twins, which has been discussed for a long time by many workers. On the other hand, it is considered to be possible from the cytological observations in various kinds of animals, that, if the twins of this third type could exist, they were from the fertilization of a ripe egg and an abnormally large second polar body by two different sperms. Rosin has recently advanced a new theory to solve statistically the problem of a third possibility of twinning, using the value of reduction introduced by Ludwig. In this paper the author has devised practical application of his theory. From the samples of twin pairs collected by the joint efforts of our Research Groups formed in 1951 for the purpose of studying twins, the author picked up 462 pairs of unlike sexed twins at random, and investigated the three human normal genetic traits of the twins and their parents, i.e., ABO blood groups, MN blood types and ear wax types (wet and dry), and analyzed the data according to Rosin's theory. As a result, significant statistics were obtained indicating the existence of a third type of twins, and the frequency of their appearance was estimated at a rate of more than 3.2 per cent in the total number of unlike sexed twins, which have been recognized a priori to be DZ