240 PHYSICAL BASIS OF HEREDITY 



eyed flies that are somatically indistinguishable from the 

 ordinary white-eyed race, yet they are the combined prod- 

 uct of several known color-producing genes. The purple 

 eye color of Drosophila is practically indistinguishable 

 from the eye colors maroon and garnet. In a word, we are 

 led again to units in the germ-plasm in our final analysis 

 rather than to the appearance of a character. 



4. Inference That Each Chakactee is the Peoduct of 



Many Genes 



We find that any one organ of the body (such as an 

 eye, leg, wing) may appear under many forms in different 

 mutant races as a result of changes of genes in the germ- 

 plasm. It is a fair inference, I think, that the normal 

 units^ — ^the allelomorphs of the mutant genes — also often 

 affect the same part. We have found about 50 different 

 factors that affect eye-color, 15 that affect body-color, and 

 at least 10 factors for length of wing in DrosopMla. 



If, then, it is a fair inference that the units in the wild 

 fly, that behave as Mendelian mates to the mutant genes, 

 also affect the same organ that the mutant gene affects, it 

 follows that many genes, and perhaps a very large num- 

 ber, are involved in the production of each organ of the 

 body. It might perhaps not be a very great exaggeration 

 to say that every gene in the germ-plasm affects several 

 or many parts of the body ; in other words, that the whole 

 germ-plasm is instrumental in producing each and every 

 part of the body. 



Such a statement may seem at first hearing to amount 

 almost to an abandonment of the particulate conception 

 of heredity, but on the contrary, the statement conveys a 

 very important idea in the modern conception of the 

 nature of the genes and the way they act. 



The essential point here is that even although each of 

 the organs of the body may be largely a product of the 

 entire germ-plasm, yet this germ-plasm- is made up of 

 units that are independent of each other in at least two 



