CHAPTER V 

 SPECIAL PROBLEMS OF MEIOSIS 



WE have seen in the preceding chapter that 

 crossing-over is an event which normally 

 happens at least once in every pair of chromosomes 

 at meiosis. If a bivalent has a chiasma -frequency 

 of 1 -0 that means that on an average two out of the 

 four chromatids undergo one cross -over, i.e. that 

 between two points situated at opposite ends of the 

 chromosome, crossing-over takes place in 50 per cent 

 of cases. Thus a bivalent with a chiasma-frequency 

 of 1-0 will have a length of 50 ' genetic units ' ; 

 similarly, bivalents with chiasma-frequencies of 2-0 

 and 3-48 will have map lengths of 100 and 174 units 

 respectively. In mapping chromosomes genetically 

 it is useful to know the total map-length in advance, 

 and this can now be calculated from the cytologically 

 determined chiasma-frequency in this simple way. 

 Table VI shows the total calculated length of the 

 genetic maps in maize and the length already known. 

 It will be seen in no case does the known length 

 exceed the total calculated length. 



In many chromosomes it appears that chiasmata 

 are as likely to be formed in one region as in another, 

 £0 that if we divide the bivalent into n short lengths 

 X microns long the chiasma-frequency of all of them 

 will be the same. This appears to be the case in the 

 long chromosomes of Vicia faba,^^^ Lilium spp.,^^* 

 Stenobothrus and Chcrtkippus ^o, 39 ^nd probably in 

 a large number of other cases. In certain organisms, 

 however, chiasmata are more or less restricted to 



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