148 GENETICS 



characters that have similar relations with white. A large 

 number have been tested, in addition to those mentioned 

 in the list. In all cases, characters that have nearly the same 

 ratios with white have together small ratios; while charac- 

 ters that have very different ratios with white have together 

 large ratios. This agrees completely with the idea that 

 we are testing. All the relations found between the ratios 

 are such as are to be expected if white is near one end of 

 the chromosome, and the other genes in the list are so 

 placed in the linear series that the near ones give small ra- 

 tios, the more distant ones large ratios. 



Of course if the gene for white were not near one end 

 of the chromosome, two other genes might be at equal 

 distances from it, but on opposite sides of it, so that they 

 would be far apart, and would therefore together have a 

 large exchange ratio. All the evidence however is that the 

 gene for white is near one end of the chromosome (though 

 not quite at its tip), so that all the genes in the list on page 

 145 lie in the same direction from white. There is evidence 

 however that the gene yellow, which has with white the 

 ratio 1.5, lies on the other side of white, close to the very 

 tip of the chromosome, as shown in figure 33, II. 



2. But if the ratios of the other genes are taken with 

 vermilion, for example (figure 33, II), we find exemplified 

 the relation just mentioned: genes that have nearly the 

 same ratio with it may behave as if they were on opposite 

 sides of it, and so have together a large exchange ratio. 

 In all these respects the relations are what is to be expected 

 if the genes are in a linear series, and their distance apart 

 determines the magnitude of the exchange ratios between 

 them. 



3. A most Important consequence of the idea that the 

 genes are in a linear series, and that crossing-over is due 

 to breaks and exchange of parts in the thread-like chromo- 

 some, is as follows. When the chromosome breaks in such 



