No. 592] THE MECHANISM OF CROSSING-OVER 



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Drosopliila contain, besides their two X-chromosomes. a 

 Y-chromosome (owing to previous mitotic abnormalities). 

 The presence of the extra homologous chromosome in 

 some way causes the X's, in some of the oocytes, not to 

 separate properly at the reduction division (presumably, 

 this is because they did not pair with each other as usual, 

 but one of them paired instead with the Y, leaving the 

 other X free to go either to the opposite or to the same 

 pole as the first X). Thus some of the eggs in which the 

 above process has occurred come to contain two X-chro- 

 mosomes, whereas normal eggs contain only one. Now, 

 it is found that in those eggs which receive both X's, no 

 interchange has taken place between them, whereas in the 

 eggs containing one X, interchange has taken place about 

 as often as usual. Hence interchange is connected with 

 whether or not the Y allows the two X's to unite and sep- 

 arate properly, i. e., interchange seems to be a result of 

 the way in which chromosomes pair and separate during 

 synapsis, and, as we have seen, if interchange occurs at 

 this period, it must be by crossing-over. 



B. The Correspondence between Separation Fre- 

 quencies and Chromosome Lengths 



In the present section still another possible test will be 

 given of the conclusions arrived at by Morgan, that the 

 factors are in line in the chromosomes, and that the order 

 in which they lie determines in a general way the relative 

 frequencies with which they separate from one another. 

 And it has just been explained that evidence for these 

 ideas is also evidence for crossing-over: that if the dia- 

 grams do represent the chromosomes and show the factors 

 in their real order, then the facts of linkage demonstrate 

 that, during synapsis, whole sections of the chromosomes 

 change places at once, i. e., cross-over. 



The second test of the validity of the chromosome dia- 

 grams is as follows: If the order of the factors shown by 

 their linkage relations, and represented in the diagrams, 

 is their real order in the chromosomes, then it would be 



