TYPES OF FACTOR INTERACTIONS 173 
pending on the dihybrid nature of the Bb7't portion of the pedigree and 
the number of W factors present. Disregarding these latter the ratio 
in F, should be 9 self black :3 black with blue tails :4 red, since the 
action of the factor T has not been determined in red birds. Of birds 
which might be classified in these categories there were actually 15 
self-black:4 black with blue tail:5 red, a very close agreement for 
such small numbers. Moreover, collecting those with factors B and b 
which should give a 3:1 ratio, the numbers are 21:7 and similarly the 
numbers for black tail and blue tail are 15:4. The ratios, therefore, 
are in close agreement. 
It should be noted that the above “blues” are not really the blue of 
the Wild Rock Pigeon but are blacks with blue tails. We can, however, 
understand how a blue pigeon might arise from mating black to white 
for it would only be necessary to employ a white of the genetic con- 
stitution bbssttW,W, to achieve this result. It is thus that complex 
cases of reversion may be explained by reconstitution of the combination 
of factors present in the original wild form. 
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