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CHAPTER 11 



concluded, then, that in twins or other indi- 

 viduals exposed to the same general environ- 

 ment as are twins, the occurrence of clubfoot 

 is the result of the environment approximately 

 68% of the time, with 71% as the approxi- 

 mate upper limit. 



Concordance-discordance studies reveal 

 only the relative contributions of genotype 

 and environment to a particular phenotype 

 (clubfoot, in the case just discussed). Such 

 studies do not teach us anything about the 

 kinds of environment involved when the geno- 

 type determines the phenotype under con- 

 sideration, nor do they teach us anything 

 about the genotypes involved when the en- 

 vironment decides the phenotype. The twin 

 studies just discussed also offer no informa- 

 tion on the effect upon penetrance of clubfoot 

 caused by environmental differences greater 

 than those found between twins reared to- 

 gether. 



In the case of tuberculosis, concordance is 

 74% for identicals and 28% for nonidenti- 

 cals. Accepting the supposition that both 

 types of twins have the same average exposure 

 to the tubercle bacillus, the susceptibility to 

 this disease is determined genetically 46-74% 

 of the time and environmentally 26-54% of 

 the time. In support of the view that the 

 extra concordance among identicals has a 

 genetic basis are the findings that concordant 

 identicals usually have the same form of this 

 disease, attacking the same place, with the 

 same severity, whereas these similarities are 

 less frequent among concordant nonidenticals. 



Paralytic poliomyelitis is 36% concordant 

 for identicals and 6% concordant for non- 

 identicals. Here, as in the case of tubercu- 

 losis, the occurrence of the disease does not 

 depend upon the infective organisms, because 

 most human beings are exposed to these 

 normally. Accordingly, the occurrence of 

 this disease depends upon the rest of the 

 environment 64-70% of the time and the 

 genotype 30-36% of the time. In the case of 

 measles, the fact that concordance is very 



high among both types of twins simply means 

 that any genetic basis for susceptibility to this 

 disease is quite uniform throughout the popu- 

 lation from which the twin samples were 

 obtained. 



The relative contributions of genotype and 

 environment to personality and other mental 

 traits may also be studied by the twin method. 

 If a metronome is run at a series of different 

 speeds, the tempo chosen as preferable will 

 be different for different people. This tempo 

 preference may be considered to be one aspect 

 of the general personality. When tests are 

 made to compare the preferred tempo of 

 identical twins, the difference in their scores 

 is found to be 7.8 of the units employed 

 (Figure 11-3). This is, as might be expected, 

 not significantly different from the difference 

 in score of 8.7 units that is obtained by test- 

 ing a given individual on different occasions. 



FIGURE 11-3. Variation in tempo preference. 

 {After C. Stern.) 



However, nonidenticals have a difference in 

 score of 15 which is significantly different, 

 being about twice that of the identicals. 

 Since siblings born at different times have a 

 difference in score of 14.5, they prove to be 

 as similar in this respect as are nonidentical 

 twins. Finally, unrelated persons show dif- 

 ferences in score of 19.5 units. Since the 

 greater the genetic similarity the smaller the 

 difference in score, it may be concluded that 



