Studies of Human Twins 



77 



identicals than identicals.) The precaution 

 of using only twins of the same sex has been 

 taken in all the twin studies discussed here. 



The concordance of AB blood type is 

 determined as approximately 64% for non- 

 identicals. What conclusions can we draw? 

 Had concordance been the same for both 

 types of twins we would conclude that there 

 is no genetic difference for AB blood group 

 in the two types of twins. The concordances 

 observed do differ, however, and do so in a 

 particular direction. The 100% concord- 

 ance for identicals is taken to mean that this 

 trait is determined genetically with a pene- 

 trance of 100% despite the environmental 

 fluctuations normally occurring between 

 identical twins. The lower percentage of 

 concordance for nonidenticals cannot be at- 

 tributed in any part to environment, since 

 an equivalent amount of environmental 

 fluctuation caused no differences in the case 

 of identicals. This lower concordance must 

 be attributed, therefore, to the differences in 

 genotype which nonidenticals have in this 

 respect. Of course, we could have predicted 

 such results from our previous discussion 

 (Chapter 5), where it was shown that AB 

 blood type is genetically determined and is 

 known to have complete penetrance. The 

 lower concordance for nonidenticals, there- 

 fore, must be due to their receiving different 

 genotypes from parents, one or both of whom 

 were heterozygous for I^ or P. 



It should be noted that it is theoretically 

 possible to obtain a result in which concord- 

 ance is lower for identicals than it is for non- 

 identicals. Such a difference in concordance, 

 if found, could be ascribed to environmental 

 differences not being equivalent for both kinds 

 of twins, being in fact greater among the 

 identicals than among the nonidenticals. 



Let us discuss the results of concordance 

 studies for some physical traits in twins 

 (Figure 11-2). Concordance for clubfoot is 

 32% for identicals, but only 3% for non- 

 identicals. The extra concordance of 29% 



(32^ 



3%) found among identicals must be 



attributed to their identical genotype. The 

 3% concordance found among nonidenticals 

 might be due entirely to similarity in genotype 

 or entirely to the environment, or to some 

 combination of these two factors. Since we 

 cannot decide this from these data, it is con- 

 cluded that in twins or other individuals 

 exposed to the same general environment as 

 are twins, the occurrence of clubfoot can be 

 attributed to genotype approximately 29% 

 of the time, with 32% as the approximate 

 upper limit. 



In the case of the identicals, 68% of the 

 time the second twin failed to have clubfoot 

 when the first twin did. This failure of con- 

 cordance is called discordance. The 68% 

 discordance between identicals is attributable 

 to differences in environment occurring be- 

 tween the partners of a set of twins. It is 



IDENTICAL 



NON-IDENTICAL 



ABO BLOOD GROUP 



CLUBFOOT 



TUBERCULOSIS 



PARALYTIC POLIOMYELITIS 



100 



33 



74 



36 



64 



28 



Figure 11-2. Discordance {umliaded) and percentage concordance {shaded) for 

 various physical traits in twins reared together. 



