Constitution of the White Leghorn Breed. 183 



zygote, barring cannot appear. Thus the outcome of the first season's 

 breeding of black with white birds, when coupled with the evidence 

 of the existence of a barring factor, B, supplied by other breeders, 

 demanded a change in view regarding the origin of the barring in 

 question; at least it required a consideration of the possibility that a 

 factor for barring might be present in one of the parent breeds used 

 in the experiments. 



If a factor for barring were present in any of the parent breeds it 

 seemed probable that it did not exist in the black 99, since experi- 

 ence has shown that the black pigment possessed by these birds would 

 cause the barring factor to be revealed even if it existed in a hetero- 

 zygous condition.* Hence it was assumed that it might be present 

 in the W. L. cf ; and it became the aim of the investigation to test this 

 point experimentally; furthermore to ascertain the behavior of this 

 type of barring in F2 and subsequent generations; also to produce, 

 by the breeding of selected birds possessing the requisite gametic 

 constitution, a barred breed, wholly distinct (at least with respect 

 to the origin of the barred pattern) from the Barred Plymouth Rocks. 

 To what extent these results have been accomplished will appear in 



the following pages. 



First, however, it is desirable to consider in some detail the prob- 

 able zygotic constitution of the black and white fowls concerned in 

 the experiments, since the expression of the factor for barring is 

 dependent as will be shown, upon the presence or absence of several 

 other factors,— especially the factors for sex and for the inhibition of 

 black pigmentation. We may therefore inquire, first, as to what 

 factors, among those with which we are especially concerned, are 

 present in the birds used in the matings already described^ 



~*For instance, it is well known that if pigment be added to the White Plymouth Rock, as a result 

 of mating with black breeds, the barring will be revealed in Fi. 



