LATENCY. 



67 



first discovered by crossing a cream-colored variety carrying the 

 factor C and a white variety carrying the factors R and P. The 

 first-generation hybrid was therefore CcRrP]). The second genera- 

 tion of this cross gave the following: 







P. 



P. 



(or crG&m) . 





1 



CCRRPP 



1 







2 CcrrPP 



2 



CCRRPp 



2 







4 CcrrPp 



1 



CCRRpj) 





1 





2 Ccrrpp 



2 



CCRrPP 



2 







1 ccRRPP 



4 



CCRrPp 



4 







2 ccRRPp 



2 



CCRrpp 





2 





1 ccRRpp 



1 



CCrrPP 







1 



2 ccRrPP 



2 



CCrrPp 







2 



4 ccRrPp 



1 



CCrrpp 







1 



2 ccRrpp 



2 



CcRRPP 



2 







1 ccrrPP 



4 



CcRRPp 



4 







2 ccrrPp 



2 



CcRRpp 





2 





1 ccrrpp 



4 

 8 



CcRrPP 

 CcRrPp 



4 

 8 







64 



4 



CcRrpp 





4 







(or cream). 

 2 

 4 

 2 

 1 

 2 

 1 

 2 

 4 

 2 

 1 

 2 

 1 



27 



28 



This gives in the second generation 27 purples, 9 reds, and 28 whites 

 or creams. 



The above illustrations will give the reader an idea of the behavior 

 of crj^ptomeric characters. These characters are not infrequent, 

 and in the early days when such cases arose they were frequently 

 reported as exceptions to Mendel's law." Their inheritance is now 

 well understood. 



II. — LATENCY DUE TO DOMINANCE OF ABSENCE OVER PRESENCE, 



Latency due to dominance of absence over presence was not 

 separately considered by Shull in the paper referred to. Generally 

 speaking, the presence of a character is dominant to its absence, 

 but there are some exceptions. Two of the best known relate to 

 horns in cattle and beards in w^heat. We do not know exactly why 

 these characters do not develop when they are in the heterozygote 

 state. It is possible that in both these cases the absence of horns 

 or beards may be due to the presence of some inhibiting factor, in 

 which case the present category of latency would be a simple case 

 of dominance of presence over absence. It seems probable, however, 

 that these characters are simply unable to develop unless represented 

 by two allelomorphs. The reasons for this assumption are rather 

 too abstruse to be given here, for to make them clear would require 

 a great deal of space, and they are also more or less speculative at 

 the present time.^ 



a See article by Shull in American Naturalist for July, 1909. 



