BRISTLE INHERITANCE IN DROSOPHILA 75 



So the general conclusion seems to stand, that after the first 

 few generations there is no close relationship between the grades 

 of the parents chosen and the grades of the children. This 

 appears to mean that in regard to extra bristles the flies in later 

 generations do not differ genetically from each other; that in 

 the later generations the variability in the number of extra 

 bristles is due mainly, if not entirely, to conditions outside the 

 germplasm. A further test of this would be to cross with normals 

 flies with many extra and flies with few extra bristles and compare 

 the results in the second generations. If the difference between 

 the many and the few extra bristles be due to genetic factors, 

 this would be shown in the Fo of the crosses. Data is given that 

 shows the distributions of extracted extra bristled flies from 

 crosses of high and low extras with normals of a different race. 

 To avoid any complication that might arise from not consider- 

 ing sexual differences, and at the same time to find evidence 

 in regard to sex linkage, four types of crosses with wild flies 

 have been made: high and low males, high and low females. In 

 table 9 the data are arranged to facilitate comparisons between 

 males and females of like grades, but by comparing alternate 

 lines the relations between high and low grades are clear. The 

 ratios and the distributions are practically alike in all cases 

 (fig. 4). In order to make the curve for high males, including 

 over 800 individuals, more easily comparable with the others, 

 these data were plotted on the basis of 600. 



In conclusion, comparisons of families in the second generation 

 of inbreeding show a tendency for higher parents to produce 

 higher children, but this is not found in the sixth or subsequent 

 generations; in single inbred lines there is found singular inde- 

 pendence of the parental and filial grades, seemingly normal 

 parents being able to produce all extra children; by the analysis 

 of crosses it is found in the ninth and tenth generations that, 

 as should be expected from the foregoing statements, high and 

 low bristle grades are genetically indistinguishable; that the 

 variability found in the late generations is apparently not due 

 to genetic factors. 



