RELATION OF GENES TO CHARACTERISTICS 185 



entire 19 genes; changing one after the other; each one 

 changed alters the eye color. 



Again consider what must be the situation in the genetic 

 system to yield one of the recessive colors: for example ver- 

 milion eye color. To produce this color, 18 of the genes 

 must be in their usual normal condition, while a single one, 

 at the point I, 33, must be altered. If in addition to this one, 

 another is altered (for example that which by itself yields 

 purple, at II, 54-5), a different color is produced. Altering 

 one gene changes the color in a certain way; altering two 

 or more changes the color in a still different way. Similar 

 relations would be found with respect to most or all char- 

 acteristics. 



8. "Multiple Factors" : The different genes on which a 

 given character depends may have qualitatively different ef- 

 fects; such is the case with the different genes that affect the 

 eye color in the fruit-fly. In other cases the different genes 

 affect the characteristic only quantitatively. Thus dimen- 

 sions often depend on many different genes. Modifying one 

 of these or substituting another for It changes the dimen- 

 sion; modifying an additional one changes it still more, and 

 so on. In such cases the character Is commonly said to depend 

 on "multiple factors." 



9. "Duplicate Factors" : In some cases a character may 

 depend on the presence of two (or more) genes of a partic- 

 ular type in such a way that both of these genes must be 

 present In order that the character shall be produced. The 

 character Is then said to depend on duplicate genes or fac- 

 tors. In the plant known as the shepherd's purse, the com- 

 mon type of pod Is triangular, but some plants have an 

 elongated oval pod. This oval pod occurs only when two 

 particular recessive genes are present In the homozygous 

 condition. Thus if the two dominant genes which produce 

 the triangular pod are A and B, while the two recessives 

 that give the oval pod are a and b, the oval pod occurs only 



