68 



E. CAKLETON MACDOWELL 



Fig. 2 Comparisons' of the distributions of inbred and extracted extra bristles 

 in corresponding generations. Upper curves, selected inbreds of the second 

 generation (broken line) and extracted extras before selection (solid line) ; lower 

 curves, selected inbreds of the tenth generation (broken line), and extracted 

 extras of the corresponding generation after selection (solid line). 



crossing becomes more marked when parents are selected for 

 several generations before being crossed, yet this is not accom- 

 panied by any increase in variability. 



4. SUMMARY 



It has been shown that the normal number of four bristles 

 is dominant to the extra bristled condition; that the simple 

 JMendelian ratio of 3 : 1 is closely approximated; that no sex 

 linkage is involved. Further, it has been shown that the flies 

 having extra bristles in the F2 of a cross have a distribution 

 lower than that of the selected flies of the corresponding gener- 

 ation, and that this difference is more marked when extra bristled 

 flies are crossed that have been selected for nine generations 



