THE MECHANISM OF HEREDITY 



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In consequence the two new chromosomes are no longer made up of 

 the same parts as the original chromosomes, but of pieces of both. If 

 we think of all the factors that lie in one chromosome as linked because 

 ordinarily they go together, in the sense that they are likely to remain 

 in that chromosome, this linkage will be disturbed, or broken, at one 

 time only in the history of the chromosomes, viz., at the time of conju- 

 gation of the pairs, when an interchange between the members of an 

 homologous pair becomes possible. 



Let me illustrate by means of two concrete cases, and by preference 

 cases that belong to the sex chromosomes, because the conditions here 

 are simpler and more convincing, and because we have more definite 

 information concerning the mode of distribution of these chromosomes 

 than of any other. 



When a male fruit fly with yellow body color and white eyes is mated 



XX XX X 



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Fig. 4. Diagram illustrating the results of crossing a yellow (stippled), white- 

 eyed male to a gray, red-eyed female of Drosophila. To the right the sex chromo- 

 somes are represented, colored in the same way as the flies. 



