76 



blood type. Then, we would determine the 

 percentage of concordance, that is, the per- 

 centage of cases where, given one twin to be 

 of the specified phenotype, the other one is 

 also. Now in the case of identical twins the 

 concordance for AB blood type is found to 

 be 100%. 



In determining concordance for non- 

 identical twins, we should usually include in 

 our data only those cases where the twins are 

 of the same sex. This is desirable because 

 the postnatal environment of twins of oppo- 

 site sex is likely to be more different than the 

 environment of identical twins — which must 

 be necessarily of identical sex on our criteria. 

 (Were both genotype and environment differ- 

 ent for the two kinds of twins, we would not 

 be able to specify the cause of a given pheno- 

 typic difference which is greater among non- 



FiGURE 11-1. Identical twins, Ira and Joel, at 

 3^ months, at 8 years, and at 16 years of age. 

 {Courtesy of Mrs. Reida Postrel Herskowitz, 

 July 14th, 1946.) 



