Studies of Human Twins 



79 



there is a genotypic contribution to this per- 

 sonaHty trait. 



Studies of twins for the mental disease 

 schizophrenia show concordance of 86% for 

 identicals and 14% for nonidenticals. How- 

 ever, it is likely that the environment is not 

 the same for both types of twins, more dis- 

 cordance being produced by differences in 

 social environment in the case of nonidenti- 

 cals than in the case of identicals. Neverthe- 

 less, in support of the view that not all the 

 concordance for identicals is attributable to 

 their similar environment, and that there is 

 some genotypic basis for concordance, are 

 two cases of identical twins who were sepa- 

 rated, grew up in different environments, yet 

 were concordant at about the same age. 



You are doubtless familiar with the fact 

 that different people score differently on I.Q. 

 examinations. We can use the differences 

 in ability to answer questions on these 

 examinations as a measure of what may 

 be called test intelligence. While the scores 

 of nonsiblings vary widely above and below 

 100, the difference between the scores of twins 

 reared together is only 3.1 for identicals but 

 is 8.5 for nonidenticals. Clearly identity in 



genotype makes for greater similarity in 

 score. Tests of identicals reared apart show 

 their scores differ by 6. In this case the 

 greater difference in environment makes for 

 a greater difference in performance of identi- 

 cals, but this is still not so great a difference 

 as is obtained between nonidenticals reared 

 together. There are, therefore, both geno- 

 typic and environmental factors affecting the 

 trait test intelligence. 



Note that in the case of AB blood group 

 we had previously discussed the nature of the 

 genetic factors involved in the determination 

 of the phenotype. We have not done this for 

 the other traits studied in this Chapter. It 

 should be re-emphasized, therefore, that 

 though the twin methods used here tell 

 whether there are genotypic differences asso- 

 ciated with the occurrence and nonoccurrence 

 of the phenotype under consideration, they do 

 not offer any information regarding the nature 

 of these gene differences. Whether or not 

 the genotypic alternatives have any capacity 

 for recombination, or whether or not they 

 recombine in a regular predictable manner, 

 cannot be determined from the data pre- 

 sented. 



SUMMARY AND CONCLUSIONS 



In human beings, the occurrence of essentially duplicate parts within an individual, and of 

 identical and nonidentical twins, offers the opportunity to test the effect of environment and 

 of genotype upon the appearance of a given phenotypic alternative. 



A considerable number of physical and mental traits has been shown to be determined 

 by the joint action of genotype and environment, sometimes the one and at other times the 

 other having the greater influence. 



The twin methods described do not study the transmissive properties of the genotypes 

 involved. They do not, therefore, reveal anything regarding the recombinational properties 

 of the genetic factors studied. 



REFERENCES 



Kallman, F. J., Heredity in Health and Mental Disorder, New York, Norton, 1953. 



Montagu, A., Human Heredity, Cleveland, World, 1959. 



Newman, H. H., Multiple Human Births, New York, Doubleday, Doran, 1940. 



Osborn, F., Preface to Eugenics, Rev. Ed., New York, Harper, 1951. 



Osborn, R. H., and De George, F. V., Genetic Basis of Morphological Variation, Cam- 

 bridge, Mass., Harvard, 1959. 



