78 



CHAPTER 6 



tributed to genotype approximately 299? of 

 the time, with 329? as the approximate 

 upper limit. 



In the case of the identicals. 689? of the 

 time the second twin tailed to have clubfoot 

 when the first twin did. The failure of con- 

 cordance is called discordance. The 68% 

 discordance between identicals is attributable 

 to differences in their environment. It is 

 concluded, then, that in twins or other indi- 

 viduals exposed to the same environment 

 that twins are. the occurrence o\ clubfoot is 

 the result of the environment approximately 

 68% of the time, with 7192 as the approxi- 

 mate upper limit. 



Concordance-discordance studies reveal 

 only the relative contributions of genotype 

 and environment to a particular phenotype 

 (clubfoot, for example, as in the case just 

 discussed). Such studies do not teach us 

 anything about the kinds of environment 

 involved when the genotype determines the 

 phenotype under consideration, nor do they 

 teach us anything about the genotypes in- 

 volved when the environment decides the 

 phenotype. The clubfoot twin studies also 

 tell us nothing about the effect upon pene- 

 trance of clubfoot caused by environmental 

 differences greater than those found between 

 twins reared together. Application of the 

 conclusions from twin studies to the gen- 

 eral population assumes that environments 

 for twins and nontwins are the same. Such 

 an assumption may be invalid. 



In the case of tuberculosis, concordance 

 is 74% for identicals and 28 % for non- 

 identicals. Accepting the supposition that 

 both types of twins have the same average 

 exposure to the tubercle bacillus, the sus- 

 ceptibility to this disease is determined ge- 

 netically 46 to 74% of the time and environ- 

 mentally 26 to 54% of the time. In sup- 

 port of the view that the extra concordance 

 among identicals has a genetic basis is the 

 finding that concordant identicals usually 



have the same form of this disease, affecting 

 corresponding organs with the same severity, 

 whereas this similarity is less frequent among 

 concordant non identicals. 



In earlier studies, paralytic poliomyelitis 

 was 36% concordant for identicals and 6% 

 concordant for nonidenticals. As in the 

 case of tuberculosis, the occurrence of the 

 disease probably did not depend upon the 

 infective organisms because most human be- 

 ings were exposed to them normally. Ac- 

 cordingly, the incidence of this disease de- 

 pended upon the rest of the environment 

 64 to 70% of the time and the genotype 

 30 to 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 

 population from which the twin samples 

 were obtained. 



The relative contributions of genotype and 

 environment to personality and other mental 

 traits can also be studied by the twin method. 

 When a metronome is run at a series of dif- 

 ferent speeds, the tempo preferred by differ- 

 ent persons is different. Tempo preference 

 may be considered to be one aspect of the 

 general personality. When tests are made 

 to compare the tempo preferred by identical 

 twins, the difference in their scores is found 

 to be 7.8 of the units employed (Figure 

 6-8). This is, as might be expected, not 

 significantly different from the difference in 

 score of 8.7 units obtained by testing a 

 given individual on different occasions. 

 However, nonidenticals have a difference in 

 score of 15, which is significantly different, 

 being about twice that of the identicals. 

 Since nontwin siblings 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 a difference 

 in score of 19.5 units. Since the greater 

 the genetic similarity the smaller the differ- 



