RELATING TO SECONDARY SEXUAL CHARACTERS. 31 



Each of these (in heterozygous condition of course) is dominant; 

 in some cases completely so, in others incompletely dominant. At 

 three different loci in the sex chromosome a dominant mutation has 

 occurred ; at three loci in other chromosomes dominant mutant changes 

 have also occurred. 



Recessives. 



White of Rose Comb bantam. Brown of Brown-breasted game. 



White of Silky. Penciling. 



White of White Rock. 



Whether the recessive white that is sometimes found in dominant 

 White Rock stock is different from both of the other recessive whites 

 is not known. There are, then, 5 or 6 recessive characters that are not 

 sex-linked and 1 recessive sex-linked character. 



Owing to the relatively large number of color dominants in poultry, 

 some unnecessary confusion has arisen concerning the relation of the 

 dominants to the wild type, and especially to other mutant characters 

 to which they are said to be dominant, in the sense, however, of being 

 epistatic. An imaginary example will illustrate this. For example, 

 if at some locus in the wild type a mutation occurred that gave a 

 dominant black (i. e., a black that shows up when one gene for it is 

 present) and at the same time this black also showed up even when 

 other recessive mutant characters were present in homozygous form, 

 then Fi birds would be black when black is crossed to such pure 

 recessive stocks. Such cases have indeed been described as dominant, 

 but a knowledge of F 2 would have shown at once the error of such a 

 system. For, if black had been a real dominant, the F 2 would have 

 given 3 blacks to 1 of the other type (such as the wild type), but if the 

 case were one of epistasis, then there would have been 9:3:3:1 

 classes in F 2 (or some modification of that ratio). In this sense, then, 

 epistasis may be defined as a result that appears when one member of 

 the pair of genes produces its effect regardless of the constitution of the 

 individual with respect to another gene (or other pairs of genes). It 

 is curious at least to note that in the case of dominant white the term 

 epistatic has been much less often used than in the case of black. 

 Theoretically the two situations are exactly alike, but because black 

 could so obviously conceal things beneath it, while white is not thought 

 of as doing so, it seemed "natural" to make such a distinction. In 

 reality it is not a question of covering up at all, but a case of a dominant 

 character (white or black) preventing other colors from appearing. 



In the case of recessive white the situation is somewhat different 

 and no one, so far as I know, has gone so far as to speak of such a white 

 as epistatic, although when the animal is white it certainly hides, when 

 completely effective, all the other effects of color-producing factors, 

 but allows them, to "show through" in some of the cases. This 

 means not that they do "show through," but that they only develop 



