TTTE SPECIAL USES OF MAN 



The Special Uses of Man 



There are four respects in which heredity in man, in spite of its 

 drawbacks, has stepped ahead of the experimental flies and flowers 

 and has been able to advance the theory of genetics. One is in the 

 study of twins. 



In man multiple births vary in frequency from 0-3 to i • 8 per 

 cent according to race. The bulk of these are derived from the 

 fertilization of several eggs at the same time. But about a quarter 



TABLE 29 



COMPARISON OF THE PERCENTAGES OF SIMILARITY 

 OF REACTION, OR CONCORDANCE, TO DIFFERENT 

 TESTS OF ONE-EGG AND TWO-EGG TWINS IN MAN 

 (BOYD, 1939; DIEHL AND VERSCHUER, 1936; 

 ROSANOFF, et. al, 1941) 



arise by the splitting of single eggs into two, four, or eight after 

 the first, second or third cleavage divisions. One-egg, monozygotic 

 or identical twins make it possible to show even more clearly than 

 in other animals such as cattle, which are less elaborately variable, 

 the relative effects of differences of genotype and environment on 

 development. One-egg twins are not merely always identical in 

 sex, blood groups, hair- whorls, taste faculties and allergies (Table 29). 

 Their finger-prints are as alike as those of the two hands of one 

 individual. They are also closely parallel in psychology, intellect 

 and susceptibility to delinquency and disease (Tables 29 and 30). 



It is the last which gives the most accurate discrimination and the 

 most unequivocal testimony. Of all diseases tuberculosis is perhaps 

 the most suitable for the test of genetic susceptibility, since infection 

 docs not lead to immunization and in some regions it is almost 



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