THE MECHANICS OF THE CHROMOSOMES 



119 



The number of chiasmata found in any one bivalent in different 

 nuclei is not constant but gives a frequency distribution with the mode 

 favoured more than would be expected on a basis of pure chance. This 

 can be explained by supposing that chiasmata show the phenomenon of 



1 2 3 U S 



Number of Cbiastnata 



LOiiy Late larly Urie f^'efc- 



U/pmne q D/plotene Diokinesis Diaklnesis phase 



D 



Fig. 56. The Cytologlcal Behaviour of Chiasmata. — ^The graph to the left 

 above gives the percentage of bivalents having different numbers of chiasmata. 

 The graph to the right shows the increase of terminal chiasmata in bivalents with 

 one, two, three, or four chiasmata. Below are the chromosomes from a diplotene 

 stage and, in the lowest row, from a late diakinesis, to show the increase in 

 terminal chiasmata. (The association on the left is a ring of four, consequent on a 

 segmental interchange for which the plant was heterozygous.) All figures relate 

 to maize {Xta = chiasmata). 



(From Darlington.) 



interference, the formation of one chiasma lowering the probability 

 that another will be found in its immediate neighbourhood.^ This is one 

 of the parallels between chiasma formation and crossing-over which 

 will be discussed in more detail later. 



The average number of chiasmata formed in different bivalents often 

 show a correlation with the length of the chromosome.^ This is only 



^ Haldane 1931. ^ Rev. Hearne and Huskins 1935. 



