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GENETICS 



Among the purple flowers are the following four 

 genotypes : 



4. SUPPLEMENTARY GENES 

 A. CASTLE'S AGOUTI GUINEA-PIGS 



An illustration of a supplementary gene that acts 

 only in conjunction with some other to bring about a 

 modification, is the pattern gene demonstrated by 

 Castle in his guinea-pigs. 



The wild gray, or "agouti," color of the hair of 

 certain guinea-pigs is due to the fact that pigment is 

 distributed along the length of each hair in a definite 

 pattern. The tip of a single hair is black followed 

 by a band of yellow, while most of the proximal part 

 which is more or less concealed by overlapping hairs 

 is a leaden color. The distribution of pigment in such 

 a pattern gives the characteristic gray, or agouti 

 color to the coat when taken as a whole. 



Castle demonstrated the separate nature and be- 

 havior of such a pattern gene when he discovered that 



