CRITERIA FOR DISTINGUISHING IDENTICAL 

 AND FRATERNAL TWINS 



By TAKU KOMAI 

 Kyoto Imperial University, Japan 



THE biological fact that there 

 are two kinds of human twins 

 and twinning phenomena is 

 now universally acknowledged. 

 No one doubts today the distinction 

 between monozygotic or identical twins, 

 who originate from a single fertilized 

 ovum by a process which is apparently a 

 kind of budding or fission in a broad 

 sense, and dizygotic or fraternal twins, 

 who come from two independent ova 

 fertilized and implanted practically at the 

 same time. A criterion for discriminating 

 these two kinds of twins from each other 

 with correctness would be of immense 

 value to geneticists, pathologists and 

 psychologists. There are in fact certain 

 features of twins which, it has been main- 

 tained by previous workers in this field, 

 serve this purpose. 



THE CHORION AND PLACENTA 



The foetal membranes, notably the 

 chorion, and also the placenta in 

 some cases, are often thought to give 

 a very reliable decision as to whether the 

 twins are monozygotic or dizygotic. If 

 each foetus is covered by a separate 

 chorion they are dizygotic, while if they 

 are covered by a common chorion and 

 provided with a common placenta, they 

 are monozygotic. No doubt this rule 

 holds true for a majority of cases. How- 

 ever, certain recent workers like Siemens 

 (19x4a, 19x5, 192-7) and v. Verschuer 



(19x7) claim to have found instances 

 which are at variance with it. Thus, 

 some twins were born with separate 

 chorions, and nevertheless show very 

 close similarity in many physical features, 

 while other twins very different from each 

 other in several characteristics were 

 covered with a common chorion at birth. 



Moreover, statistical data, according to 

 these authors, suggest the existence of 

 such exceptional twins. There is a simple 

 method of calculating the approximate 

 number of monozygotic twins among a 

 given twin population, from the number of 

 different-sex twins found in it, which is 

 called Weinberg's differential method. 

 This method is based on the assumption 

 that the sex of one of the dizygotic twins 

 is determined independently of the other; 

 if this is the case, there must be about the 

 same number of dizygotic twins among the 

 same-sex twins as there are different-sex 

 twins, and the rest of the same-sex twins 

 are monozygotic. By this method, the 

 percentage of monozygotic twins has been 

 estimated at from x6 to 36 (according to 

 v. Verschuer 's figure); this exceeds more or 

 less the percentage of monochorionic 

 twin-births, which is 14 to x6. It is 

 thus likely that a small percentage of 

 monozygotic twins are born with separate 

 chorions. 



Dahlberg (19x6) criticizes the above 

 contention of Siemens and states that the 

 abnormal cases mentioned by the latter 



