146 GENETICS 



2. At this period the two chromosomes are side by side, 

 in conjugation. 



3. And each Is seen to be a slender elongated thread, 

 having thickenings at intervals, in linear series (figures 36 

 and 37). 



4. The breaking apart and exchange of parts therefore 

 occurs in these long slender threads. 



5. Possibly therefore the differences in the frequency of 

 separation of different parts of the thread are due to the 



A BCDEFGH 

 T • • • — • 1 • • • 



-•-♦- 



^wab c vm '"B 



Figure 33. Diagrams to illustrate the arrangement of genes in linear 

 series. I, Genes A to H (see text). II, The order of the genes pro- 

 ducing yellow (y), white (w), abnormal abdomen (a), bifid wings 

 (b), club wings (c), vermilion eye (v), miniature wings (m), rudi- 

 mentary wings (r), and bar-eye (B), according to the theory of 

 linear arrangement of genes. 



fact that some parts are close together on the thread, oth- 

 ers far apart. If the chromosome breaks in but one or two 

 places, as appears to be the case, parts that are far apart 

 would become separated more frequently than parts that 

 are close together. 



Represent the chromosome by a line on which there 

 are consecutive thickenings. A, B, C, . . . to H, represent- 

 ing genes (figure 33, i). Then if a break occurs anywhere 

 in the chromosome it will separate the extreme genes A and 

 H. But to separate any two adjacent genes, as E and" F, 

 a break must occur precisely in the short stretch that lies 

 between them. Any break that separates two genes that 

 are near together will separate others lying farther apart. 

 The result of these relations is that the farther apart in the 



