10 EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 



vertical. The close fit of this line to the observed data shows that the 

 relation 



Percentage of abnormal females = 1.5 X percentage of abnormal males 



may be taken as approximately describing the average observed con- 

 dition. 



Owing to the impossibility of describing the intensity of abnormality 

 in quantitative grades, we can not give a formula for showing its sexual 

 relation. Tables 2 and 3 show that there is such a relation. The ques- 

 tion as to whether both wings or only one shall be abnormal is also a 

 part of this same problem of the variation of the intensity of the abnor- 

 mality. We have seen that when a female is abnormal she will in 74 

 per cent of the cases be so abnormal that both wings will be affected, 

 while only 62 per cent of her abnormal brothers will be abnormal in both 

 wings. 



THE RANGE OF VARIATION OF ABNORMALITY INCLUDES "NORMAL" VENATION. 



One other point is to be noted. The intensity of abnormality ranges 

 all the way from cases in which there is almost as much abnormal vena- 

 tion as normal down to a barely discernible devia- 



Table 5 

 tion from normality. We have, then, in studying 



the inheritance of abnormal venation, the serious 

 difficulty that a just indiscernible abnormality may 

 be present.* Such a fly would be recorded as nor- 

 mal. Table 5 suggests that they would be more like- 

 ly to occur in families in which the percentage of 

 abnormal offspring is low, for as such percentage 

 decreases the percentage of abnormal individuals 

 which are abnormal on both sides (C. S. ) decreases. In other words, 

 there is an increasing percentage of abnormal flies which have the abnor- 

 mality so reduced that in at least one wing it can not be seen. Hence, 

 presumably, there is an increasing percentage of flies which have the 

 abnormality reduced in both wings to a point just below visibility. 

 These will be more common among males than among females, because 

 the intensity of the abnormality is less in male than in female wings. 

 Whether this alone accounts for the fact that a smaller percentage of 

 brothers are visibly abnormal than of sisters is a question to which it is 

 difficult to give an answer. 



• * M ^l ^°i this be true aIso of the s P° tted condition in certain mammals? A guinea- 

 pig still behaves as a spotted animal even if the spots are reduced until only the eyes 

 remain affected. If the variation goes still further we would have an animal germi- 

 " 1 t S ^°" te somatlcally s P otless - We would then say that the spotted condition is 



