156 GENETICS 



attached to one of the autosomes. In such a case the char- 

 acteristics dependent on the genes in this piece of X change 

 their method of inheritance in later generations; they are 

 now distributed to offspring by the method of autosomal 

 inheritance, instead of by sex-linked inheritance; for now 

 they are attached to an autosome. It is therefore possible 

 by study of the inheritance in later generations to determine 

 what genes are present in the piece of X that is broken off. 

 Such determinations have been made in many cases. 



The results of these studies show that the order of the 

 genes shown on such maps as figure 38 is correct. When a 

 small piece of the end of the X-chromosome is broken off, 

 such study as we have described shows that it contains the 

 genes for yellow body, white eyes, and the other genes near 

 the tip of the chromosome. If a longer piece of X is broken 

 off, it contains more of the genes — those extending farther 

 toward the middle of the chromosome. Such studies, with 

 the X-chromosome and with the autosomes, have been made 

 on a large scale in Drosophila; they confirm throughout the 

 order of the genes shown on the maps, the order originally 

 determined by study of the different exchange ratios of the 

 genes, as described in the foregoing pages. 



Thus, in sum, all the tests to which the matter has been 

 subjected agree with the conception that the chromosome is 

 composed of separable genes, having different effects on 

 development and characteristics; that these genes are ar- 

 ranged consecutively, like beads on a string; and that the 

 different exchange ratios of different pairs of genes are due 

 to the different distances of the genes one from another. 



This conception is subject to test in many other ways that 

 cannot be described here. The consecutive arrangement of 

 the genes results in many diverse mathematical relations, 

 which must hold in the results of experimentation, if the 

 consecutive order is correct. It may be stated that the re- 

 sults of literally hundreds of thousands of experiments 



