VI 



REVERSION 



67 



which a bird is white, and a black modifier (B), in the 

 absence of which a coloured bird is blue. The original 

 black barb contained 

 both of these factors, 

 being in constitution 

 CCBB. The fantail, 

 however, contained 

 neither, and was con- 

 stitutionally ccbb. 

 The Fi birds produced 

 by crossing were in 

 constitution CcBb, 

 and being heterozy- 

 gous for two factors 

 produced in equal 

 numbers the four 

 sorts of gametes CB, 



Cb, cB, cb. The results of two such series of gametes 

 being brought together are shown in the usual way in 

 Fig. n. A blue is a bird containing the colour factor 

 but lacking the black modifier, i.e. of the constitution 

 CCbb, or Ccbb, and such birds as the figure shows appear 

 in the F 2 generation on the average three times out of 

 sixteen. Reversion here comes about in F 2 , when the 

 redistribution of the factors leads to the formation of 

 zygotes containing one of the two factors but not the 

 other. 



cb| 

 cbJ 



1 



BLACK 



EDI 



BLACK 



E3 



BLACK 



|Cb| 



|cb1 



alp 



Cb 1 

 cBJ 



mm 



1 cB I 



|cb| 



cBI 



CbJ 



cB 

 cB 



cB 

 cb 



1 cb 1 



|cb| 



cb'^^ 



M 



Wfm 



cb 

 cB 



cb 

 cb 



BLACK 



Fig. 11. 



Diagram to illustrate the appearance of the rever- 

 sionary blue pigeon in F 2 from the cross of 

 black with white. 



