THE GREAT RELAY RACE 159 



Mendel did not hit upon linkage, because it fortunately so happened 

 that the determiners of the seven characters (page 444) with which ho 

 dealt were each located in separate chromosomes, of which there are 

 known to be seven pairs in garden peas. This was a happy accident, 

 for if Mendel had chanced upon genes linked together in a single 

 chromosome, he might never have been able to establish the law of 

 independent assortment, which is so essential in determining the 

 Mendelian ratios. 



In mitosis it sometimes happens, however, as shown by the sub- 

 sequent breeding results, that chromosomes emerge which contain a 

 different combination and arrangement of genes than that in the 

 originals from which they came. In other words, linkage is broken 

 up. The way this comes about is as follows. During the process of 

 the preparation of the germ cells for sexual union (mciosis), as has 

 been repeatedly observed, the maternal and paternal chromosomes in 

 each pair of egg or sperm come to lie close together side by side. 

 They may even twist around each other. This intimate contact of 

 homologous chromosomes from the two parents is called synapsis. 

 It will be recalled how later the still entire chromosomes separate or 

 unwind from their mates and migrate to opposite poles of the germ 

 cell, during the unique reduction division, thus forming two new cells 

 each containing but half the normal number of chromosomes in each 

 cell. This means that either the maternal or the paternal chromosome 

 of each pair is missing in the resulting daughter cells, while the end 

 result of ordinary mito.sis, or cell division, is the production of two 

 new cells, each with a complete equipment of chromosome pairs 

 representing the maternal and paternal contributions. 



After synapsis, the two chromosomes in each pair may separate and 

 go their different ways with all their genes linked together exactly as 

 they were before intimate contact with each other, or during synapsis 

 they may stick temporarily together and then later break into frag- 

 ments and become reassembled in a new relationship, with a part of 

 a paternal chromosome attached the supplementary part of a mater- 

 nal chromosome. When such an interruption of linkage occurs it is 

 termed crossing-over. It is as though, following an ardent embrace, 

 Jack's head should be found perched on Jill's shoulders, and in 

 exchange, Jill's head should turn up on Jack's shoulders. That this 

 extraordinary kind of performance actually does happen with the 

 chromosomes has been amply demonstrated over and over by observ- 

 ing the ratios in which the offspring appear following a dihybrid cross. 



