BIOLOGY IN HUMAN AFFAIRS 



litter of coal-black progeny. The saying probably became 

 current simply to generalize on the undeniable fact that 

 the cub of a fox is another fox, while the foal of a mare is 

 a horse; but as a genetic principle it is lame and impotent. 

 It would not have gained popular approval had people 

 appreciated the logical consequences of the common 

 observation that a child never wholly resembles one of its 

 two parents; for the obvious deduction from this fact is 

 that the physical basis of heredity must be a set of discrete 

 units gathered together in the germ cells, and that the laws 

 of heredity must be the principles which govern the dis- 

 tribution of these units during the maturation of the germ 

 cells and the fertilization of the egg. When such an idea 

 did take form in the brain of a man who could not rest 

 until he had tested its validity, the science of genetics was 

 born. Gregor Mendel was the man. 



Mendel was a physicist by training. As such he was 

 keenly aware of the value of dealing with the lowest 

 possible number of variables, of controlling all experi- 

 mentation carefully, and of searching for statistical 

 relationships among the observations recorded. He chose 

 to study inheritance in the garden pea rather than light 

 or sound; but the choice did not make him forget the 

 rigorous discipline of earlier days. He selected pairs of 

 varieties to be crossed which differed but by a single 

 striking character. He made sure that he was dealing with 

 uniform material by self-pollinating plants of each type 

 and studying the resulting progeny. The variety which 

 bred true for its distinctive trait was used; the variety 

 which did not breed true was discarded. He then made 

 seven crosses, each involving only one pair of contrasting 

 characters; and followed each lone character through 

 several generations produced by self-fertilization. Previous 

 hybridization experiments had come to naught because the 

 varieties used as parents had differed by hundreds of 



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