GENETIC STUDIES ON FLOUR-MOTH 435 



B. Reversal of dominance 



Reversal of dominance by sex has been called sex-limitation 

 by Morgan ('14). This phenomenon has been demonstrated for 

 horns in sheep by Wood ('05), for supernumerary mammae in 

 swine ('12), and color in Ayrshire cattle ('16) by Wentworth, 

 and for color in the butterfly, Colias philodice, by Gerould ('11). 



Variations in dominance are frequent and are due to environ- 

 ment, age, or genetic constitution. 



The present studies show a perfectly clear case of reversed 

 dominance due to a simple Mendelian difference. Sooty is a 

 simple dominant to type, while black is a simple recessive. In 

 the homozygous black moths, however, sooty acts as a recessive. 

 The dihybrid ratio, therefore, approximates 9 sooty, 3 type, 1 

 sooty black, and 3 black. Other ratios are discussed in the text, 

 and it is suggested that reversal of this sort may be expressed 

 by reversing the symbols denoting the factors. Thus heterozy- 

 gous sooty is Ss.BB while heterozygous sooty black is sS.bb. A 

 possible explanation for the reversal of dominance in this case 

 is suggested under the discussion of the physiology of color 

 production. 



C. Analysis of a case of continuous variation 



The separation of sooty base from type was at first impossible, 

 as moths in single fraternities varied continuously from the most 

 pronounced sooty to undoubted type. In later generations the 

 segregation became sharp in some fraternities, but in others the 

 intergradation persisted. A hereditary variation which darkened 

 slightly the base of the wings, thus showing a slight tendency 

 towards sooty, was probably responsible for some of the con- 

 fusion. By far the greatest difficulty, however, resulted from 

 the variation known as dark mid-area or dark. This variation 

 has an effect somewhat the reverse of sooty base. It shows 

 much intergradation with type, but probably depends upon a 

 simple Mendelian difference. The nature of the reverse effect 

 of sooty and dark is further discussed in the following section. 



