234 The Inheritance of Wmg Colour in Lepidoptera 



but unfortunately the parental values cannot be shown as the insects 

 were unobtainable. 



Family '16 A (Fig. 8) shows yet another cross of the same nature. 

 The yellow parent is again very dark, and the homozygous yellows are 

 in excess of the hybrids. In this family the females have been kept 

 divided from the males, and the orange colour- values of their fore wings 



>» 



o 



c 

 o 



3 



0) 6 



[1-3: 0-7] 



Colour- value of 



hybrid parent 



5 Colour-units 



[4-9 : 4-6] 



Colour-value of 



lutea parent 



Fig. 8. (Cf. Fig. 21.) Curve showing frequency distribution of the orange 

 and yellow colour-values of '16 A, hybrid {lutea x gross.) x lutea. 



The orange values of the females alone are shown by means of a dot-and- 

 dash curve. 



have been shown by a line of dots and dashes, which differs in no essential 

 from the same curve for both sexes (see p. 219). Reference to Fig. 21 

 shows that the excess of yellows over hybrids is greater than in the three 

 preceding cases, there being at least 64 yellows to 20 hybrids, or a ratio 

 of 76: 24 (per cent.). 



Now supposing the yellow colour develops in the variety lutea owing 

 to the loss of an inhibitor /, from the white type //, then this excess of 

 yellows might of course be accounted for on the hypothesis of another 

 factor X, a deepener, which has no visible effect upon yellow ii, but which 

 turns the pale hybrids li into deep yellows. If for instance the yellow 

 parent of '16 A had been Xx, then half the hybrid offspring would have 

 been liXx, and half lixx. Those carrying X and I would be changed 

 into deep yellows, and the result would be 75 per cent, deep yellows, and 

 25 per cent, pale yellows or whites. The ratio observed in this family 

 ('16 A) is very close to the result expected on this hypothesis. 



