Abstract
III. Further Relations between Chromosomes and Heredity. IN examining the chromosomes for a stage when “crossing-over” might be possible, we turn naturally to the time when the members of each pair come together. This occurs once in the history of every germ-cell. In many accounts it has been shown that the members of each pair come to lie side by side throughout their length. Even more interesting is the fact that just prior to this union the chromosomes have spun out into long, thin threads. There are also several detailed accounts showing that at this time the two chromosomes of each pair may actually twist about each other in one or more turns (Fig. 16). They then come to lie side by side and appear as a single thread that shortens preparatory to entering upon the first maturation division. Here, apparently, we find realised a condition that might make interchange possible between the members of a pair of chromosomes, for if the threads fuse where they cross each other and the ends on the same side unite, the interchange of pieces will be accomplished. From the nature of the case it would be almost impossible to demonstrate that the twisted threads do break and make new unions at the crossing point. It is true that there are certain later stages that lend, perhaps, some support to the view that breaking and reunion have occurred, as Janssens has pointed out, but it cannot be claimed that this evidence does more than give, on such an assumption, an account consistent with certain con figurations he describes. Here the case must rest for the present. The genetic evidence is clear and far in advance of what the cytologist is able to supply. But, nevertheless, it is very important to find that, so far as the cytological evidence goes, it furnishes a great many of the facts essential to the kind of process that the genetic evidence calls for.
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MORGAN, T. The Mechanism of Heredity1. Nature 109, 312–313 (1922). https://doi.org/10.1038/109312a0
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DOI: https://doi.org/10.1038/109312a0