OLD TYPES AND NEW 



163 



NUMBER OF 

 ZYGOTE IN 

 FIGUHE 49 



Duplex for both color (large circle) and pigment (large cross) 

 Duplex for color (large circle) but simplex for pigment 



(large cross) 

 Simplex for color (large circle) but duplex for pigment 



(large cross) 

 Simplex for both color (large circle) and pigment (large 



cross) 



2-5 

 3-9 



4-7-10-13 



b. Castle's Agouti Guinea-pigs 



An illustration of a supplementary factor that acts 

 only in conjunction with some other to bring about 

 a modification, is the pattern factor demonstrated by 

 Castle in his guinea-pigs. 



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

 tain 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 factor when he discovered 

 that it is transmitted independently of pigment, which is 

 necessary to bring it to expression. He showed that 

 upon crossing a solid black guinea-pig, unquestionably 

 possessing pigment but no "pattern," with a white 



