RELATION OF GENES TO CHARACTERISTICS 1 95 



at all beyond a certain stage. By ingenious methods Dem- 

 erec ^ has recently proved that if only a few of the cells of 

 a developing embryo are deficient for a certain gene, those 

 few cells fail to develop, though the rest of the body does 

 develop. This was demonstrated for deficiencies of many 

 different genes in the X-chromosome of Drosophila. In only 

 one of the many cases tested was deficiency for a certain 

 part found not to be lethal, though even in this case it 



a 

 b 



Figure 43. Method of testing for a deficiency. The chromosome 

 b, carrying the recessive gene r, is brought into the same fertilized 

 egg with the chromosome a, which does not carry the recessive. If 

 the recessive r is manifested in the individual developed from the 

 egg, this shows that the chromosome a has a deficiency at the region r. 



caused weakness. In the case of the white eye gene at I, 1.5, 

 and in many other loci, deficiency prevented development. 

 All together, it is clear that genes are active throughout 

 development, some beginning early, others seemingly later, 

 and that almost all genes are necessary in order that full 

 development shall occur. Changing one or more genes may 

 cause from the beginning a change in the chemistry and 

 physiology of the organism, altering its course of develop- 

 ment and so giving rise to changed characteristics. 



What Kinds of Characteristics are Affected by 



Genes f 



The question as to what kinds of characteristics are af- 

 fected by genes has been much discussed. Some have sup- 

 posed that only a few characteristics depend on genes: only 

 certain superficial matters, such as colors, and the forms of 



