STURTEVANT 



143 



Table 25 



on 



Facet counts fro?/!-—- 



Table 26 



Facet coiints from 



BB 



may really be single type plus normal 

 allelomorph. This possibility requires 

 further experimental investigation. 



ARE MUTATIONS IN GENERAL DUE TO 

 UNEQUAL CROSSING OVER? 



One of the first problems raised by 

 the discovery of the nature of bar re- 

 version is as to how widespread may 

 be the phenomenon of unequal cross- 

 ing over. One direct test has been at- 

 tempted, making use of Muller's 

 method of testing for the frequency 

 of occurrence of new lethal mutations. 

 Females were made up that carried one 

 wild-type X chromosome and one X 

 with' the mutant genes scute (locus 

 0.0), echinus (5.5), crossveinless 

 (13.7), cut (20.0), vermilion (33.0), 

 garnet (44.4), and forked (56.8). Such 

 females were mated to males carrying 

 all the mutant genes named. In such 

 matings it is possible to detect prac- 

 tically all the crossing over that occurs 

 in the X chromosome, except that to 

 the right of forked (about 13 units). 

 Counts were made from individual fe- 



males, in order to make sure that they 

 carried no lethals. Forty-one wild- 

 type daughters (non-crossovers) were 

 tested from such matings, to see if the 

 non-crossover X chromosomes carried 

 new lethals. The only lethal that oc- 

 curred was in a paternal (i.e., scute 

 echinus crossveinless cut vermilion 

 garnet forked) chromosome. Thirty- 

 eight double-crossover daughters and 

 one triple-crossover (i.e., a total of 79 

 crossings over) were also tested; and 

 again no lethals occurred in any of the 

 maternally derived chromosomes, 

 though there were two doubtful cases 

 of new lethals in the chromosomes de- 

 rived from the multiple-recessive 

 fathers. While this experiment was 

 done on a small scale, it furnishes no 

 indication that crossover chromo- 

 somes are more likely to contain new 

 lethals than are non-crossovers. 



There is, however, another kind of 

 evidence that argues against any gen- 

 eral applicability of the unequal-cross- 

 ing-over explanation of mutation, 

 namely, the cases in which mutations 



