LINKAGE, CROSSING-OVER, AND THE GERM PLASM 417 



linked characters discovered in DrosophUa, but by no means the last. 

 Soon after the discovery of the white-eye mutant, there appeared in 

 typical stock characterized by gray body color a single male mutant 

 distinguished by yellow body color; and this character was found to be 

 inherited exactly after the manner of white eyes. In other words, it 

 is sex-linked, and therefore must have its gene in the X-chromosome. 

 As time went on, many new sex-linked characters appeared as mutants, 

 always noted in males, and these characters had to do with all sorts 

 of bodily characters. Several of these were new eye colors (vermilion, 

 ruby, prune, garnet); others had to do with eye shape or eye texture 

 (furrowed, bar eye, and small eye); others, with wing size and shape 

 (broad wing, club wing, cut wing, vestigial wing) ; others, with bristles 

 (scute, singed, forked); others, with body color (tan, sable); and some 

 were lethal characters. Altogether, over sixty definite sex-linked mu- 

 tant genes, with their allelomorphs, have been found to be sex-linked 

 and therefore must have their loci in the X-chromosome. 



Now, during the twenty years of breeding millions and millions 

 of drosophilas and examining them for signs of new hereditary char- 

 acters, some hundreds of other mutations were noted and their modes 

 of heredity studied. This latter great collection of mutant characters 

 showed no sex linkage, so could not be assigned to the X-chromosome. 

 They must in all probability be located in the autosomes, as the rest 

 of the chromosomes are called. But which of the remaining chromo- 

 somes carry the various non-sex-linked genes is not definitely known. 

 An important fact, however, soon came to light, namely, that prac- 

 tically all of these non-sex-linked-genes fall naturally into two groups 

 of nearly equal size. The basis for this distribution of genes into two 

 groups is this: that some of the characters appear together more often 

 than not, while other characters appear apart (i.e., in separate indi- 

 viduals) more often than together in the same individual. This pro- 

 cess of classifying characters results in two large groups of characters 

 of nearly equal size, each rather more numerous than the sex-linked 

 group. The two large classes of genes that seem to hang together in 

 heredity more often than they go apart have come to be assigned to 

 chromosomes II and III (Fig. 78) respectively, and are known as the 

 second and third linkage group. At present it is not possible to dis- 

 tinguish between Chromosome II and Chromosome III, for they are 

 of the same size and shape; but the point is that there are only two 

 other pairs of large chromosomes in DrosophUa and only two large 

 linkage groups besides the sex-linked group. What more natural. 



