BIOLOGY IN HUMAN AFFAIRS 



ance whether they contained genes BB (or Bb') or not. 

 Moreover, it was discovered that sometimes two genes, let 

 us say AA and BB, had to be present together for the pro- 

 duction of a detectable character. Thus a white sweet 

 pea AAbb, crossed with another white sweet pea aaBB, 

 produces a purple hybrid AaBb, since both the A gene and 

 the B gene are required for the manufacture of the purple 

 pigment. This discovery, by the way, cleared up a hoary 

 old biological puzzle — namely, why certain crosses show 

 reversion to the type characteristic of some bygone 

 ancestor. 



The numerous minor discoveries made during this period 

 need not concern us here, since the objective of the first 

 part of this essay is the groundwork of present-day genetic 

 philosophy. It is necessary, however, to call attention to 

 the general concept of gene behavior which slowly took 

 form in the minds of genetic investigators. From the first 

 it did not seem probable that even such an apparently 

 unimportant gene as, for example, the one which differ- 

 entiates brown eyes from blue in the human race, could 

 have only a single function. Were this true, an organism 

 would be merely a genetic mosaic, a piece of animated 

 tiling. And this proved not to be the case. Genes are units 

 in inheritance, but are not units in development. Develop- 

 ment is a coordinated affair which one may think of as the 

 progressive unfolding of the organism as a whole. In this 

 unfolding a single gene has manifold duties, and numerous 

 genes contribute their quotas toward the fulfillment of a 

 seemingly simple task. For instance, one must assume that 

 the mammalian eye is the result of the combined activity 

 of hundreds of genes; one must also assume that each of 

 these genes, though primarily concerned with eye develop- 

 ment, may have minor effects on many other organs. 



It took only five or six years from the beginning of the 

 genetic renaissance, which came with the rediscovery in 



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