200 GENETICS 



these are often called fraternal twins. Methods have been 

 discovered by which it Is possible to determine to which class 

 any pair of twins belongs.^ 



Identical twins in man are of enormous interest; they 

 supply exactly what is required for discovering what char- 

 acteristics depend on genes. They are two individuals that 

 are made throughout from the same set of materials, from 

 the same set of genes. Every single gene of one has its exact 

 duplicate in the other. The two therefore have the same 

 heredity; the differences between them are due to environ- 

 ment alone. They show how like or how diverse will be two 

 human beings that do not differ in their genes, in their 

 heredity, although they may differ in the conditions of their 

 lives, in the experiences they undergo. Identical twins thus 

 form one of the most interesting and illuminating of the 

 phenomena that nature presents to us. They yield the key 

 to that ancient question, the relative role of nature and 

 nurture: of heredity and environment. 



The existence of the "two-egg" twins, with their diverse 

 sets of genes, for comparison with the one-egg twins, having 

 identical sets of genes, completes what is required for dis- 

 covering just what resemblances and differences in human 

 beings are due to identity and diversity of genes. The con- 

 ditions to which the two members of a pair of twins are 

 subjected are as much alike in the two-egg twins as in the 

 one-egg twins. Therefore we can be certain that any differ- 

 ences between the two types of twins are due to differences 

 in the genes, not to diversities of environment. 



Comparison of one-egg twins (having the same genes), 

 with two-egg twins (having different genes) : Identical 

 twins are extraordinarily alike in all physical respects. They 

 are always of the same sex. Their faces and figures are so 

 much alike that it is difficult to distinguish one from the 

 other. They are alike in eye color, hair color, skin color. 

 Their facial features are of the same form and relative 



