432 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



The ratio of 27, 9, 9, 9, 3, 3, 3, i shows clearly that the three factors 

 independently segregate and are all three concerned hi the determi- 

 nation of the characters of the fur. A fourth factor, a pattern factor, 

 is often present that further complicates the factorial analysis. Usually 

 the self-color dominates the pattern, but certain special patterns are 

 dominant over self-color. 



These two examples for animals are sufficient to illustrate the 

 nature of Mendelian factors and their workings. Numerous other 

 factors have been discovered. Castle, for example, found a factor 

 associated with the occurrence of brown pigment in guinea pigs. 

 Some rabbits have the pigment distributed evenly over the body; 

 others have it in the eye only. These conditions are allelomorphic to 

 each other, E (extension) being dominant over e (restriction to eyes). 



Inhibiting factors are distinguished, the presence of which prevents 

 the appearance of a charr cter represented in the germ plasm. Lethal 

 factors result hi the loss of something necessary for the life of the 

 individual. Modifying factors change the expression of a character 

 that depends on another gene. These and various other types of 

 factors have been discovered by the large school of neo-Mendelians 

 now so actively at work. 



