46 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



have to be taken into account. Moreover, by means 

 of this principle it has been possible not merely to 

 predict the outcome of some complex combinations, 

 but also to plan and breed new combinations success- 

 fully, much as one may sometimes foresee the nature 

 of a new organic chemical compound that has been 

 created by synthesis. 



The Mendelian principle has already been put to 

 use in relation to certain hereditary characters in 

 human beings. An instructive example relates to 

 the color of the eyes. The study of the transmission 

 of eye coloring has been much confused by the 

 difficulty of properly classifying the colors with 

 respect to their purity. A feature of fundamental 

 importance is whether the pigment lies on the back 

 surface of the iris, as in pure blue eyes, or on both 

 front and back surfaces, as in pure brown or black 

 eyes, and in a less regular way in green and hazel 

 eyes. Careful study shows that in heredity the eyes 

 with double pigment layers dominate the eyes with 

 single pigment layers. In terms of the Mendelian 

 theory the pure blue (single pigment layer) eye is 

 recessive, the pure brown (double pigment layer) 

 is dominant. The law of Mendelian inheritance 

 now makes it possible to explain why two parents 

 with pure blue eyes never have a brown or black- 

 eyed child, why the children of a pure black-eyed 

 and a pure blue-eyed parent have black eyes, and 

 why parents with hybrid dark eyes (with mixed 

 pigment layers) may have blue-eyed children. If 



