186 Bulletin No. 155.— 1913. 



the Barred Plymouth Rocks permits the assumption that the W. L. 

 d^ cf are homoz3"gous (BB) while the 9 9 are heterozygous (Bh) 

 for this character. This circumstance would maintain the barred 

 pattern under conditions of equilibrium in both sexes in successive 

 generations; and logically we cannot assume otherT\4se if our experi- 

 mental birds belong to pure-bred stock. On this assumption, the 

 complete zygotic formula for the W. L. c^ would be C2B2f2l2, while 

 that of the 9 would be CzBbFfh, since the 9 is assumed to be 

 heterozygous for both female sex and barring, and homozygous for the 

 inhibiting factor. On the basis of these assumptions (which, it must 

 be fully understood, are for the moment merely assumptions, used to 

 frame a working hypothesis) we may now turn to a more detailed 

 consideration of the special cases. 



A. Discussion of the Special Cases. 



Cases 1,2,3 and 4- — On the basis of the assmned zj^gotic formulae 

 previously stated, what is the expected result of crossing the \V. L. cf 

 with the black 99? The W. L. cf forms onl}^ one type of gamete, — 

 CBfl, while the black 99 form two types, CbFi and Cbfi. The 

 mating may then be represented 



^ CBfl ' CBfl X 

 9 Chfi ' CbFi — 



cf" d" C^BbfJi, white 

 9 9 C2BbFfIi, white 



In other words, the first cross between the W. L. cf and the black 

 9 9 giyes birds that are all white and heterozygous for the barring 

 factor and for I. It has been stated in the description of the experi- 

 ments that a few Fi birds put up one or two barred or partly barred 

 feathers. This may be explained on the grounds that the dominance 

 of the inhibiting factor, /, was not complete when, as in Fi, it existed 

 in a heterozygous or simplex condition. Where a little black was 

 permitted to show, there it filled out the pattern of a barred feather. 

 When the black was inhibited to a still greater degree, the pigment 



