TYPES OF FACTOR INTERACTIONS 173 



pending on the dihybrid nature of the Bb Tl portion of the pedigree and 

 the number of W factors present. Disregarding these latter the ratio 

 in FZ 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 6 

 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 n W n 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. 



