Phenotypic Effects of Gene Action 



ABO BLOOD GROUP 



CLUBFOOT 



TUBERCULOSIS 



PARALYTIC POLIOMYELITIS 



IDENTICAL 



77 

 NON-IDENTICAL 



64 



28 



figure 6-7. Discordance (unshaded) and percentage con- 

 cordance (shaded) for various physical traits in twins reared 

 together. 



is likely to be more different than that of 

 twins of the same sex. (If the environment 

 differed for the two kinds of twins, one 

 would not be able to specify whether the 

 environment or the genotype was the cause 

 of a phenotypic difference that is greater 

 among nonidenticals than identicals.) Only 

 twins of the same sex are used in the twin 

 studies discussed here. 



The concordance for ABO blood type is 

 approximately 64% for nonidenticals. Had 

 concordance been the same (64% or 

 100%) for both types of twins, we would 

 conclude that there is no net genetic or 

 environmental difference for ABO blood 

 group in the two types of twins. The con- 

 cordances observed do differ, however, and 

 do so in a particular direction. Because of 

 this difference the 100% concordance for 

 identicals must mean that this trait is deter- 

 mined genetically with a penetrance of 

 100% despite the environmental fluctua- 

 tions normally occurring between identical 

 twins. Since an equivalent amount of en- 

 vironmental fluctuation caused no differences 

 in the case of identicals, the lower percent- 

 age of concordance for nonidenticals cannot 

 be attributed in any part to environment. 

 This lower concordance must be attributed, 

 therefore, to the differences in genotype 

 which nonidenticals can have in this respect. 



Of course, we could have predicted such a 

 result from the previous knowledge that 

 ABO blood type is genetically determined 

 and is known to have complete penetrance. 

 The lower concordance for nonidenticals, 

 therefore, must be due to their receiving 

 different genotypes from parents, one or 

 both of whom were heterozygous for l A 

 or P. 



It is theoretically possible to obtain a re- 

 sult in which concordance is lower for iden- 

 ticals than it is for nonidenticals. Such a 

 difference in concordance could be ascribed 

 to environmental differences being greater 

 among the identicals than among the non- 

 identicals. 



Consider the results of concordance stud- 

 ies for some physical traits in twins (Figure 

 6-7). Concordance for clubfoot is 32% 

 for identicals, but only 3% for nonidenticals. 

 The extra concordance of 29% (32% minus 

 3% ) found among identicals must be attrib- 

 uted to their identical genotype. The 3% 

 concordance found among nonidenticals 

 might be due entirely to similarity in geno- 

 type or entirely to the environment, or to 

 some combination of these two factors. 

 Since we cannot decide from these data, we 

 conclude that in twins or other individuals 

 exposed to the same environment that twins 

 are, the occurrence of clubfoot can be at- 



