Phenomena of Inheritance 101 



which is independently heritable, and many recent students of 

 heredity hold a similar view. But it is evident that there is not an 

 exact one to one correspondence of inheritance units and adult 

 characters. Many different characters may be determined by a 

 single unit or factor ; for example, all the numerous secondary 

 sexual characters which distinguish males from females may be 

 determined by the original factor which determines whether the 

 germ cells shall be ova or spermatozoa. 



Multiple Factors. — On the other hand two or more factors may 

 be concerned in the production of a single character. In many 

 cases among both plants and animals the development of color 

 appears to depend upon the presence in the germ cells and the 

 cooperation in development of at least two factors, viz. (i) a pig- 

 ment factor for each particular color, and (2) a color developer. 

 When both of these factors are present color develops, when 

 either one is absent no color appears. 



Such cases have been described for mice, guinea-pigs, and rab- 

 bits as well as for several species of plants. Bateson and Pun- 

 nett found two varieties of white sweet peas which were appar- 

 ently alike in every respect except the shape of their pollen grains, 

 one of them having long and the other round pollen. But when 

 these were crossed a remarkable thing occurred for the progeny 

 "instead of being white were purple like the wild Sicilian plant 

 from which our cultivated sweet peas are descended." This is 

 apparently a typical case of reversion and its cause was found in 

 the fact that at least two factors are necessary in this case for 

 the production of color, a pigment factor R and a color devel- 

 oper C. One of these was lacking in each of the white parents, 

 their gametic formulas being Cr and cR respectively, but when 

 these two factors came together in the offspring a purple-flowered 

 type was produced with the zygotic formula CcRr. These F t 

 plants produced colored and white F 2 plants in the proportion of 9 

 colored to 7 white and the colored forms were of six different 

 kinds (Fig. 33). For the production of these six colored forms 



