180 MULTIPLE FACTORS 



in a mouse already carrying b and therefore white. 

 There are also reverse cases where, in the presence of 

 B, a and A produce no different effect and thus a 

 ratio of 12AB + aB:3Ab:lab is obtained. 



Departures from the 9:3:3:1 ratio different from 

 those given above result if one factor for a character 

 is dominant and another recessive. For example, 

 there is a white race of fowls that is dominant and 

 another white race that is recessive. There are two 

 cocoon colors in silkworm moths that have this same 

 relation. A cross of a dominant white to a recessive 

 white gives a ratio of 13:3. Here, instead of the 

 recessive classes resembling each other, so that a 

 9:6:1 or 9 : 7 ratio is produced, both the 9AB and 3Ab, 

 since they contain the dominant white (A), re- 

 semble the one ab containing the recessive white 

 (b), and only the 3aB appear colored. In this 

 case the effect of the white does not happen to be 

 cumulative, but there is no reason why factors which 

 differ as to dominance should not have a cumulative 

 action; if they did, a 3 :10 :3 ratio would result. 



Cases belonging to any of the types given above 

 show modified ratios if the dominance is incomplete, 

 for then the heterozygous classes are intermediate in 

 character between the others. Consequently, in 

 these cases, the different classes are usually not 

 as easy to distinguish from one another as if domi- 

 nance were complete, for the character differences now 

 separating the classes are smaller. In such cases, 

 especially if the character is appreciably influenced 

 by environmental conditions, the individuals in an}^ 



