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 conjugation 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
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 chromosomes are represented, colored in the same way as the flies.
