THE PRIMARY COLOR-FACTORS OF LYCHNIS AND 



COLOR-INHIBITORS OF PAP AVER RHOEAS 1 



George Harrison Shull 



The frequency with which the presence of hereditary characters 

 is dominant over their absence naturally suggests that inhibiting 

 factors may be operating when the reverse relation appears to 

 exist, as when the hornless character of polled cattle dominates 

 over horns, and the "smooth" character over the "bearded" in 

 wheat, oats, etc. Some writers (Davenport, Bateson, Punnett) 

 have even taken the extreme view that dominance is in all cases 

 a criterion of "presence." That this position is untenable I have 

 shown several years ago (Shull ii, 12), and Castle (2) also opposes 

 such an idea, calling attention to Wood's well known sheep hybrids 

 (Wood 15), in which the horned condition is dominant in the male 

 and hornlessness in the female offspring from the same cross, as a 

 proof that no such sweeping generalization is permissible. It may 

 be granted, however, that presence is usually dominant, and that 

 the dominance of the apparent absence of a character is probably 

 in most cases, but not in all, the dominance of an inhibiting factor 

 over its own absence. It is only necessary to keep the mental 

 reservation that in any single instance of a putative inhibitor 

 another hypothesis is always available, namely, that the gene for 

 the character that is supposed to be inhibited, when existing singly 

 as in the heterozygote, may be nearly or quite incapable of reaching 

 the threshold of visible expression. 



Both of the characters mentioned above by way of example 

 the polled condition in cattle and the lack of long awns in wheat 

 are structural characters. When a c^r-character is inhibited, the 



1 Under the title "Inhibiting factors in Lychnis and Pa paver" this paper was 

 read before the Botanical Society of America, Washington, D.C., December 28, 

 191 1. The change of title and slight changes in the text have been rendeied 

 necessary by the discovery that the purple-flowered male parent of family 10201 

 discussed below was probably heterozygous in both the primary factors for color. 

 This discovery in no wise affects the general considerations presented in the paper 

 as read, but it withdraws Lychnis dioica for the present as an example of domi- 

 nant white. 



Botanical Gazette, vol. 54] I I2 ° 



