642 INHERITANCE OF A FLUCTUATING CHARACTER 



The Fi Result of the Cross between " With " and the Wild Stock. 



The above classifications were made preparatory to a cross be- 

 tween these two Hnes. The Fi flies from the cross of "without" 

 wild flies to "with" flies of the mutant stock were in the main inter- 

 mediate in grade between the two parental types (Table I, and Curve c, 

 Fig. 2) . The mode of the Fi was at Grade IV. There were very few 

 flies that did not show a faint pattern (Grade I = 1.2 per cent). 

 At the other extreme there were no flies that were as dark as Grade 

 VIII and only a very few (1.1 per cent) as dark as Grade VII. A 

 comparison of the Fi distribution with that of each parent (Fig. 2) 

 shows that, as judged by the means, modes, and extent of overlap, 

 the Fi population resembles the wild parent more closely than the 

 "with" parent. "With" is classified as a "partial" dominant since 

 it is known to be the mutant type and the Fi flies show considerable 

 effect of the "with" gene. 



The Fofrom the Cross of "With" to Wild. 



The F2 results (Table I, Curve d, Fig. 2) gave a range of grades 

 that included the faintest "withouts" of the wild grandparent and 

 the darkest "with" of the other grandparent. In shape the F2 

 curve presented its main mode at Grade III and an indication of 

 another mode at Grade VII. If our assumption is correct that the 

 "with" stock differed from the wild because of a single mutant dif- 

 ference, the Fo population should be composite, one quarter being 

 like the wild parent, one quarter like the "with" parent, and two 

 quarters Hke the Fi type. On this basis an expectation for the F2 

 that should result from the given Pi stocks and the known Fi dis- 

 tribution, can be readily calculated (Table I). 



The curves of Fig. 2 show that the observed F2 result (Curve d) 

 is in good accord with the expectation (Curve e). The observed F2 

 differed from that calculated in that there was slightly too great a 

 proportion in Grade VII (which corresponds to the mode of the 

 "with" stock) and also a similar excess in Grades II and III (which 

 corresponds to the bulk of the wild stock). 



It should be noted that had "contamination of genes" been acting, 

 the F2 should have deviated from the calculated Fo in the direction 



