THE FACTORIAL HYPOTHESIS 219 



absence scheme it may, however, be difficult to think 

 of the small letters as anything but absences. It 

 may, therefore, be helpful to represent the same 

 formulae with other letters. 



If the original comb was single, which in fact is 

 the type of comb of the wild bird from which the 

 domesticated races have come, a dominant muta- 

 tion from A to A' gave rise to a rose comb; another 

 dominant mutation from the wild type that changed 

 B to B' gave rise to a pea comb; a third but recessive 

 mutation that changed C to C' gave rise to a "comb- 

 less" comb. The normal allelomorphs would be 

 represented by the same letters without the primes. 

 The formulae (in simplex) for the combs would then 

 be as follows: 



Wild type (single) ABC 

 Rose A'B C 



Pea A B'C 



Combless ABC' 



The walnut comb that appears when pea is bred 

 to rose is, of course, the double dominant form A 'B'C. 



If it seems desirable to use letters that give a clue 

 to the name of the factor for which they stand, either 

 of the next alternatives would cover the case under 

 discussion. In the second of these the small letters 

 are not absences, but only the recessive allelomorphs. 



Wild type (single) P R C or p'r'C 



Rose PR'C orp'R'C 



Pea P'RC orPVC 



Combless P R C' or pVc 



