76 



CHAPTER 6 



gotic origin of twins. Assuming the absence 

 oi mutation. an\ single difference in such 

 traits would prove the twins aonidentical. 

 (On this basis, twins oi' opposite sex are 

 classified immediately as aonidentical.) Of 

 course, two such differences would make the 

 decision practically infallible, since two mu- 

 tations in genes governing the limited num- 

 ber of traits being compared in a pair of 

 identical twins would be so rare as to be 

 beyond any reasonable probability of occur- 

 rence. 



Identification of monozygotic twins re- 

 quires the same phenotypic comparisons. 

 The larger the number of traits for which 

 no genetic difference is demonstrated, the 

 greater the probability that the twins are 

 identical. When the number of traits serv- 

 ing to test the genotypes of twins is suffi- 

 ciently large, it becomes nearly certain that 

 they would have shown one or more differ- 

 ences had they been dizygotic in origin. 

 Failure to show any such difference, then, 

 may be attributed to identical genotypes de- 

 rived from a single zygote. 



Let us outline the procedure one might 

 actually follow in using twins to study the 

 relative roles of genotype and environment 

 in producing specific traits. The objective 

 is to score separately the percentage of iden- 

 tical and nonidentical twin pairs reared to- 

 gether in which one or both siblings have 

 the trait under consideration. Suppose one 

 wished to study the ABO blood group in 

 this respect. One would determine the per- 

 centage of concordance, that is, the percent- 

 ages of identical and of nonidentical pairs 

 in which both members of a pair have the 

 same phenotype. In the case of identical 

 twins concordance for ABO blood type is 

 found to be 100%. 



In determining concordance for noniden- 

 tical twins one usually scores only pairs in 

 which the twins are of the same sex. This 

 convention is necessary because the post- 

 natal environment of twins of opposite sex 



FIGURE 6-6. Identical twins, Ira and Joel, at 

 3% months, at 8 years, and at 19 years of age. 

 (Courtesy of Mrs. Reida Postrel Herskowitz 

 July 14, 1946.) 



