94 Plant Genetics 



of control that there could have been no experimental 

 error. This new fact demanded explanation, for with 

 such chromosomes as TS and ts it would be impossible 

 to obtain a tall wrinkled individual as long as the 

 individuality of the chromosome is maintained. When 

 chromosomes were examined with the modern lenses 

 they were found to show all kinds of tangled contortions 

 during reduction division, and accordingly the scheme 

 shown in fig. 26 was devised. These five stages repre- 

 sent phases that allelomorph chromosomes may go 

 through during reduction division. Two allelomorph 

 chromosomes, which normally would be side by side 

 (i) may at times come to lie across one another (2). 

 In this, position the middle regions of the chromosomes 

 are in contact and are conceived of as fusing (3). The 

 spindle fibers from each pole then lay hold of this com- 

 pound chromosome and the pull comes in the direction 

 of the arrows shown in the figure. This results in the 

 break indicated in 4. Finally, two new chromosomes 

 separate from the old compound chromosome, as indi- 

 cated in 5. Thus T is linked with s, and later, when 

 two such chromosomes are brought together in crossing, 

 the result is a tall wrinkled individual. In the same 

 way dwarf smooth individuals may appear. 



This is known as crossing over, and the literature in 

 reference to it is extensive. It is on the basis of this 

 conception that certain geneticists claim to be able not 

 only to locate given determiners on their proper chro- 

 mosomes, but also to tell exactly on what parts of the 

 chromosomes these determiners are located. 



This mechanism readily explains the phenomenon of 

 coupled and antagonistic characters without involving 



