MULTIPLE FACTORS 227 
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:1ab 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 8aB 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 above show 
ratios further modified if 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 any 
