STURTEVANT 



127 



mozvgous bar. In all cases the mothers 

 had been heterozygous for forked,^ 

 which lies 0.2 units to the left of bar, 

 and for fused ^ which lies 2.5 units to 

 the right of bar. All six reversions 

 represented crossovers between forked 

 and fused, though the total forked 

 fused crossovers constituted less than 

 3 percent of the number of individuals 

 examined. Sturtevant and Morgan also 

 reported that experiments in which 

 bar entered only through the males 

 had failed to give any reversions, 

 though no numerical data were re- 

 ported. The present paper is based on 

 the results of a more detailed study of 

 the relations first shown by Sturtevant 

 and Morgan (1923). 



MUTATIONS AND CROSSING OVER 



The results from homozygous bar 

 females, that were reported by Sturte- 

 vant and Morgan, were from females 



-\- R f 

 of the constitution -7-^5 — —. A more 



J B + 



efficient type of experiment is that in 

 which females of the constitution 



r T, r are mated to forked bar fused 



males. Table 1 (first row) shows the 

 results obtained from an extensive 

 series of this type. In the second row 

 of table 1 are given the results from 

 mating a few females of the above 

 constitution to forked fused males. 



Table 1 



fBh 



9 XfBf^S (1st row) orff^ $ (2nd row) 



* In this and the following tables "0" signifies non-crossover classes; "1," classes resulting 

 from crossing over in the first region, etc. 



The mutant females that appeared 

 in these experiments were, of course, 

 all heterozygous for whichever allelo- 

 morph (bar or round) they received 

 from their fathers, but in tables 1 to 



2 Forked is a recessive bristle modifica- 

 tion. Locus 56.8 in the X chromosome (see 

 Morgan and Bridges 1916). 



3 Fused is a recessive venation character. 

 Its locus is at 59.5 in the X chromosome (see 

 Morgan and Bridges 1916, Lynch 1919). 



18 this paternal gene is ignored and 

 the females classified according to the 

 maternal gene. In all doubtful cases 

 (including both double-bars), the clas- 

 sification was checked by raising off- 

 spring from the mutants, since there is 

 sometimes difficulty in classifying sin- 

 gle individuals, a difficulty that can be 

 removed by examination of a series of 

 specimens of a given constitution. 

 It will be observed that seven of the 



