OF MUTANT CHARACTERS. 145 



vant, 1912), this same procedure was applied to the non-sex-linked 

 mutations. The cytological work of Miss Stevens had shown that 

 there were at least three autosomes in Drosophila, and it was expected 

 that a group of linked genes would be found to correspond to each of 

 these. At this time there were only two non-sex-linked mutations 

 black and pink whose inheritance had been worked out by Morgan 

 and whose behavior was definitely known to be Mendelian and normal. 

 The next point was to establish the relation of these mutants to each 

 other in inheritance. This was done by raising an F 2 from the cross of 

 black by pink. The F 2 ratio was quite clearly that of independent 

 inheritance, since it approximated 9 : 3 : 3 : 1, with the double reces- 

 sive present in as large numbers as expected (Sturtevant, February 1, 

 1912). Sturtevant soon showed by means of back-cross tests that 

 there was no appreciable linkage between black and pink. Provision- 

 ally, black and pink were assumed to be in separate chromosomes 

 the second and the third. The second chromosome is arbitrarily that 

 chromosome which carries the gene for black, and any other genes that 

 may be found to be linked to black; similarly, the third chromosome is 

 defined as that chromosome which carries the gene for pink, and all 

 other genes found to be members of the linkage group containing pink. 

 Soon after the black pink F 2 had given the first autosomal inde- 

 pendence, an F 2 between black and curved demonstrated the first 

 autosomal linkage. As soon as this linkage was observed (March 4, 

 1912) definite plans were made to test the linkage relations (chromo- 

 some and locus) of all the autosomal mutants thus far found, making 

 full use of the back-cross method. (See Bridges and Sturtevant, 1914, 

 p. 205). By the middle of July it was known as a result of these tests 

 that besides black and curved, purple, vestigial, balloon, blistered, 

 jaunty, and arc were in this second chromosome. 



LOCUS OF BLACK. 



The locus of black was taken as the base of reference in the mapping 

 of these other genes. Since curved was the first mutant known to be 

 in the second chromosome with black, its locus determined the direction 

 along the chromosome which was to be defined as " to the right' ' (black 

 curved). The loci of all the other mutants just mentioned were later 

 found to lie on the same side of black as curved does, so that black was 

 the locus farthest to the left, and the natural zero-point of the map. 

 Black is now the real base of reference in the mapping of the entire 

 second chromosome, and all other genes are plotted in relation to it, 

 either directly in the case of those genes nearby (dachs, jaunty, purple, 

 vestigial, etc.), or indirectly by being located with reference to certain 

 important genes (star, curved, speck, etc.), whose positions with regard 

 to black have been so well established that they in turn can safely be 

 used as secondary bases. 



