CHAPTER XX 



THE NATURE OF GENES 



When a pair of alternative characters, such as pigmentation 

 and albinism, is involved in a cross, we assume that the 

 gamete which. transmit5~T>ne of the alternative conditions 

 differs structurally from that which transmits the other and 

 that this structural difference is the cause of their difl'erent 

 powers of transmission. By the study of linkage relations 

 we find that the structural difference is confined to a par- 

 ticular linkage group, in mice and rats to the group which 

 also includes the factor for pink-eyed dilution. If we adopt 

 the chromosome hypothesis, we locate the structural differ- 

 ence in a particular chromosome and suppose that it exists 

 in a definite region (or locus) of that chromosome. Each 

 structurally different state of a locus is called a gene. The 

 color gene shows the alternative forms which we call C and 

 c. With all the residual heredity unchanged, C will cause the 

 development of full pigmentation, while c will leave the skin 

 unpigmented. For information as to what C and c are, we 

 may consult the biochemists, who have devoted considerable 

 attention to the chemical processes involved in pigment for- 

 mation. Wright (1917) after an exliaustive review of the 

 chemical evidence concludes (1) '*that melanin (pigment) 

 is produced by the oxidation of certain products of protein 

 metabolism by the action of specific enzymes, (^2) that the 

 reaction takes place in the cytoplasm of cells probably by 

 enzymes secreted by the nucleus, (3) that various chromo- 

 gens are used, the particular ones oxidized depending on the 

 character of the enzymes present, and finally that hereditary 

 differences in color are due to hereditary differences in the 

 enzyme element of the reaction." The final conclusion is of 

 particular interest. It indicates that the gene C is concerned 

 in enzyme production. Wright offers a provisional hypolhe- 



177 



