OF MUTANT CHARACTERS. 



159 



the two distances required to determine approximately the position 

 of|the gene for free-vein. As in the previous experiment, only those 

 flies which actually show the free-vein (about a quarter of the whole 

 number) can be used in the calculation. As shown in the last section 

 of table 20, there were 224 flies with the free vein, of which 93 or 41.5 

 per'cent were cross-overs. When double crossing-over is taken into 

 account, this value indicates that the gene is very far away from black 

 in the second chromosome, probably 50 or more units away. At this 

 time it was known that the dominant mutant streak was in the left end 



Table 21. — Pi, free-vein cf X streak 9 ; B.C., Fi streak free-vein 9 X loild cf. 



of the chromosome, and while its portion was not accurately known, 

 it was at least 30 units to the left of black. The right end of the chro- 

 mosome was well mapped for at least 50 units, so that while the gene 

 for the free vein might possibly be in either end, it was more probably 

 in the right end. Both of these possibilities were tested. A free- 

 veined male was mated to a streak female, and one of the Fi streak free- 

 veined females was out-crossed to a wild male, which corresponds in 

 this case to the double recessive. Five of the eleven free-veined flies 

 were streaked (table 21), which free crossing-over means that the free- 

 vein gene is not in the left end of the chromosome and is therefore in 

 the right end. This fact was shown directly by the test with speck, 

 whose locus is in the right end of the chromosome. A free- veined 



