228 SABRA COLBY TICK. 



i 



Earlier experiments (Sturtevant, '13) have shown that there 

 is about 30.7 per cent, of crossing over between white and 

 vermilion. Since there is 25.3 per cent, crossing over between 

 vermilion and barred the factor for barred must lie close to white 

 (approximately 5.4) or else very far from white. That the latter 

 is the case is shown by the large percentage of crossing over 

 between white and barred (42.8 per cent.). 



Other work done in this laboratory, largely as yet unpublished, 

 shows that, when distances as long as this between white and 

 barred are involved, the chromosomes often break at two points 

 and re-unite. As a result of this double-crossing over, the non- 

 cross over classes are increased and the cross over classes are 

 diminished (Diag. ir). The amount by which the percentage 

 42.8 is less than the percentage 30.7 + 25.3 is therefore an indi- 

 cation of the amount of double crossing over that has occurred. 

 Sturtevant ('13) has located certain factors on the X-chromosome. 

 Vermilion (B} is given as 30.7. The experiments in this paper 

 giving 25.3 per cent, crossing over between vermilion and barred 

 enable us to locate the new factor barred approximately at 56 

 (3-7 + 2 5-3) (without considering the double crossing-over be- 

 tween white and vermilion or that between vermilion and barred). 



DOMINANCE. 



In these experiments whenever flies were heterozygous for 

 barring they showed the barring without exception. In this 

 sense the dominance is constant, in that a fly which fails to show 

 the barring cannot transmit it. As was stated before the homo- 

 zygous females have a narrower bar than the heterozygous 

 females. That it is possible to pick out at sight the two different 

 flies is verified by the following experiments. 



