No. 609] 



THE THEORY OF THE GENE 



519 



4. Inference that Each Character is the Product 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 affect 

 the same part. By way of illustration I may state that 

 we have found about 50 eye-color factors, 15 body-color 

 factors, and at least 10 factors for length of wing in 

 Drosophila. 



If then, as I have said, it is a fair inference that the 

 units in the wild fly that behave as Mendelian mates to 

 the mutant genes also affect the organ in question, it fol- 

 lows that many and perhaps a very large number of 

 genes are involved in the production of each organ of the 

 body. It might perhaps not bo a very iii-oat exaggeration 

 to say that every gene in the uci-iti plasm affects every 

 part of tlie body, or, in otlier words, that tlir whole gfirna. 

 ])lasm is instriiiiiental in iirodiiciii,!;- each and every part 

 of tliebody. 



Such a statement may seem at first heariiiu' to amount 



of lieredity. But in reality it is only a conclusion llased 

 on fact. The essential pnnit hn-r that erru nJthnHfih 

 each of the organs of the Ixxhi niaii he Jarf/rli, drpcudmt 

 on the entire germ plasm for its developrnt ui . get this 

 germ plasm is made up of independent pairs of }(infs. 



5. Evidence that Genes have a Real Basis in the derm 



Plasm 



In 1906 Bateson, Saunders and Punnett found that cer- 

 tain pairs of characters in sweet peas did not behave in- 

 dei)endently of each other, but tended to stay together, 

 or to ket'p apart, in succeeding generations according to 

 the way they entered the cross. Every year more cases 

 of linkage are found, so that there can be little doubt 



