THE FACTORIAL HYPOTHESIS 273 
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 
formule: 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 formule (in simplex) for the combs would then 
be as follows: 
Wild type (single) A BC 
Rose A/BC 
Pea AB’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 RC or p’r’C 
Rose P RC or p’R’'C 
Pea PRC or P’r’C 
Combless PRC’ or p’r’c 
