Chapter 13 

 MISCELLANEOUS LINKAGE TOPICS 



Cytogenetic Proof of Crossing Over 



That genetic crossing over was caused by breaks and reahgn- 

 ments between chromatids at pachytene was not always reaUzed ; 

 and even after the analogy between crossing over and chiasmata 

 was pointed out, it lacked experimental proof for a considerable 

 time. 



That the genes might be located in the chromosomes was 

 suggested by Sutton in 1903 who, however, had no conception 

 of crossing over. The possibility that genes in one chromosome 

 could exchange with their alleles in the homologous chromosome 

 was suggested at about the same time by de Vries and others, 

 but the exact mechanism of meiosis was not understood at that 

 time and the possible method of bringing about such exchanges 

 was largely a matter of speculation. A few years later, evi- 

 dence of the mechanism of such exchanges was furnished by 

 Janssens, who demonstrated chiasmata. The possible explana- 

 tion of all linkage phenomena on the basis of Janssens 's chias- 

 mata was suggested by Morgan in 1911, who pointed out that if 

 the genes were arranged in a linear order on the chromosome, 

 the genes in a chromosome which came from the maternal 

 parent could lie during synapsis next to their alleles in the ho- 

 mologous chromosome derived from the paternal parent. As the 

 result of this pairing of alleles, the exchange of groups or blocks 

 of genes on one chromosome with similar groups on the homol- 

 ogous chromosome could occur and undoubtedly did occur at 

 about the time of chiasma formation. This theory agreed with 

 the then known observations, but, for twenty years thereafter, 

 conclusive proof was lacking. There was no proof because the 

 two homologous chromosomes are identical when viewed under 

 the microscope, and although such chiasmata can be interpreted 

 as due to an exchange of homologous segments, there is no way 



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