390 



EVOLUTION, GENETICS, AND EUGENICS 



The ratios given are the theoretical ratios for a trihybrid Mendel - 

 ian experiment, and the actual results have closely approximated these. 

 Asa matter of fact, sixteen albinos appeared, and it is not possible, 

 except by breeding, to tell one kind from another. Breeding each 

 with, for example, Silver-fawn would readily repeal the differences; 

 for the F, generation would all be of the color that is masked by the 

 lack of C in these albinos. In the language of Johanssen there is only 

 one albino phenotype, but there are four albino genotypes. Similarly 

 m experiments (i) and (4), which have just been described, the indi- 

 viduals are all Black (phenotypically identical), but that they are not 

 genotypically alike is clearly shown by inbreeding them. In experi- 

 ment (i) we get only individuals of the four color types, while in 

 experiment (4) we get, in addition to the four color types, four albino 

 t3^es. 



castle's guinea pigs 



Professor W. E. Castle was one of the first zoologists to use Men- 

 del's methods. He soon discovered that in the determination of the 

 coat characteristics of guinea pigs at least three sets of factors were 

 necessary, as follows: 



C= colored fur 



c= albinism (absence of C) 



5= short fur 



5= long fur (recessive to S) 



i?=rosetted fur 

 r = smooth fur (absence of R) 



An example will show how these factors segregate: 

 P Colored, Short, Smooth X Albino, Long, Rosetted 



iCSr) (csR) 



F, 100 per cent Colored, Short, Rosetted (CSr-csR) 



F, 



Colored 



Short 



Rosetted 

 27 



Albino 



Long 



Rosetted 



Colored 



Long 



Rosetted 



9 



Albino 



Long 



Smooth 



Colored 



Short 



Smooth 



9 



Albino 



Short 



Rosetted 



9 



Colored 



Long 



Smooth 



3 



Albino 

 Short 

 Smooth 

 3 



