THE FACTOR HYPOTHESIS AS APPLIED TO ANIMALS 271 



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

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

 As a 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 reveal 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 

 in experiments (1) and (4), which have just been described, the indi- 

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

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

 ment (1) we get only individuals of the four color types, while in 

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

 types. 



FACTORIAL ANALYSIS OF COAT COLOR IN SWINE 



Some breeds of swine have a red coat color, as the Duroc Jerseys. 



Other breeds have a sandy yellowish color. Still others are white. 



There are other colors, but they may be ignored for present purposes. 



Wentworth has shown that there are two genotypicaily distinct, 



though phenotypically identical, sandy types, the factor analysis being 



as follows: 



SSTT= red 



SStt = sandy 



55 TT— sandy 



55// = white 



If this analysis be correct, when the two different sandy genotypes 

 (SStt and ssTT) are interbred, the result would be as follows: All of 

 the F : individuals would be red. These, when interbred, give the fol- 

 lowing F 2 phenotypic ratio: 



9 5T=red 

 3 St = sandy 

 3 sT= sandy 

 1 5/= white 



The phenotypic ratio is, therefore, 9 red to 6 sandy to 1 white, which is 

 a rare modification of the 9:3:3:1 dihybrid ratio, and is due to the 

 fact that the phenotype, sandy, consists of two different genotypes in 

 equal numbers. 



