TYPES OF MENDELIAN HEREDITY 45 



speak of factors for eyes or for legs, we really mean 

 factor-differences which can produce effects only in 

 the eye, the leg, or other regions of the body. In 

 other cases the expression of a factor-difference may 

 not be limited to one region but may produce a 

 different effect in different regions; for example, a 

 gray white-bellied mouse, which differs from the 

 yellow mouse by only a single factor, is lighter than 

 yellow on the under side, but darker on the upper side. 



///. By the Influence of Other Factors 



Analogous also is the fact that certain factor- 

 differences produce a visible effect only when they are 

 in company with a particular complex of other heredi- 

 tary factors. Thus, a fly with the factors for ver- 

 milion eyes can not be distinguished from one with 

 the factors for pink eyes if both contain, in addition, 

 the factors for white eyes, for the factors for white 

 allow no other color to develop. Again, it is obvious 

 that without the factors necessary for the develop- 

 ment of a given character, no factors merely deter- 

 mining special modifications of that character can 

 have any effect. In other cases, the effect of a given 

 factor may not be entirely suppressed, but greatly 

 changed, if certain other factors in the hereditary 

 complex are changed. Thus, in flies which already 

 have the factor for vermilion eyes, the factor for 

 purple eyes produces an eye still lighter than ver- 

 milion, but in flics containing the normal allelomori)h 

 of the factor for vermilion, the factor for purple pro- 



