174 MULTIPLE FACTORS 



with reference to each other, but by their different 

 linkage values with other factors. 



An example of a cross, involving at the same time 

 two factor-differences which have similar effects, is 

 Nilsson-Ehle's cross of dark brown oats having two 

 dominant factors for dark glumes with white-glumed 

 plants having the two recessive allelomorphic factors 

 for hght color. The expected F2 ratio is 9 double 

 dominant dark browns (AB) : 3 hght browns having 

 the first recessive and the second dominant (aB) : 3 

 fight browns having the first dominant and the 

 second recessive (Ab) : 1 double recessive white (ab) . 

 Since the two factor-differences produce similar 

 results, however, the fight browns, aB and Ab, are 

 indistinguishable; counting these two classes to- 

 gether, a 9 : 6 : 1 ratio results. The 9 double dominants 

 were distinguishable from the 6 single dominants, 

 the pigment being dark brown in the 9 cases where 

 both factors for dark glumes were present and both 

 factors for fight glumes absent, but onl}^ light brown 

 in the 6 cases where one light and one dark factor 

 were present. Similarly the 1 double recessive, 

 having both fight and no dark factors, was much 

 fighter even than the 6 light browns. This result 

 may be described by saying that the effects of the 

 factors for dark and for light were afi cumulative 

 or summative, two darks producing a blacker pig- 

 ment than one, and two lights a paler color than one. 



In many cases, multiple factors do not give results 

 that may, in the above sense, be called cumulative. 

 For example, if a white-flowered sweet pea (ab) 



