RELATING TO SECONDARY SEXUAL CHARACTERS. 23 



In the Fj birds at least 11 classes may be distinguished, and some 

 of these appear composite. For 3 factors the maximum number of 

 possible classes (including heterozygotes) is 27. We can recognize at 

 least 11 F2 classes amongst the females alone, and a few others are 

 doubtfully present in the males. 



In favor of the view that the heterozygous classes are here different 

 from the homozygous, the following evidence may be utiUzed: 



(1) The Fi birds are entirely different from either parent and they 

 are heterozygous for all the factor differences between the two types. 

 The only alternative explanation for the intermediate condition of Fi 

 would be that each race carries one or more completely dominant 

 factors. But the latter view is improbable because more of each parent 

 type would then be expected in the F2 generation. 



(2) In the Fa generation the Fi type is not as frequent as would be 

 expected on the view that the heterozygotes could not be distinguished, 



E. Back-Cross of Fi 9 to Sebeight cf . 



It is possible to add, now, while this paper is passing through the press 

 (June 1919), the results of a back-cross of 4 Fi females to a Sebright 

 male carried out during the summer of 1918. The birds being now 

 mature their permanent colors are evident. Making the back-cross 

 in this direction is much less advantageous than the reciprocal de- 

 scribed above, because the Sebright contains most of the dominant 

 color factors. The group of birds obtained appeared to be less varia- 

 ble in color than those from the other back-cross, and one can see at a 

 glance that more of them approach the Sebright type; some quite 

 closely. 



All of the males are hen feathered, as expected. No evidence was 

 found that two types of males exist, which would have been expected 

 if the two types noted in F2 had any hereditary significance. If, then, 

 as the F2 results suggest, two factors for hen-feathering are present 

 both are dominant, and no genetic distinction is found between in- 

 dividuals in which one or both of the dominant factors are duplex 

 or simplex. 



There were 9 adult hens and 10 hen-feathered cocks. An attempt 

 is made below to refer them to their corresponding F2 classes. 



F. Review of the Heredity of the Color of the Plumage of 



Poultry. 



In poultry there are perhaps more different colors and color-patterns 

 than in any other species of domesticated animals. The genetic work 

 has advanced far enough to show that many of the differences depend 

 on Mendelian factors. It is probable that, in addition to the main 

 factors, there are many contributory, minor, or modifying factors that 

 give the finer details to "show birds." 



