loo Outline of Genetics 



The inference was that in these same cultures there could 

 never appear a tall plant with white flowers nor a dwarf 

 plant with red flowers, for if there were linkage, and the 

 chromosome were the indivisible unit in inheritance, it 

 would be impossible for tallness and redness to become 

 separated. As a matter of fact, it was soon recognized 

 that these 'impossible" individuals did actually occur. 

 Small numbers of tall whites and dwarf reds regularly 

 appeared among the same cultures in which the linkage 

 of tallness and redness had been demonstrated, and the 

 work had been done under such conditions of control 

 that there could have been no experimental error. 



This new fact demanded an explanation, for with 

 such chromosomes as TR and tr it would be impossible to 

 obtain a tall white individual so long as the individuality 

 of the chromosome was maintained. When chromosomes 

 were examined with the modern lenses they were found 

 to show all kinds of tangled contortions during the 

 reduction division, and accordingly the scheme shown 

 in fig. 1 8 was devised. These five stages represent phases 

 that an allelomorphic pair of chromosomes may go 

 through during reduction division. This pair of chromo- 

 somes, which would normally lie 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 compound 

 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 indicated in (5). 

 Thus T becomes linked with r, and later, when a mating 



