120 



AN INTRODUCTION TO MODERN GENETICS 



clearly found in organisms with unlocalized chiasmata, but where the 

 chiasmata are localized there might be a similar relation between 

 frequency and the length of the chiasma forming segment, but this can- 

 not easily be determined since there is no way of telling how long the 

 chiasma forming segment is. In organisms with random chiasmata the 



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Fig. 57. Relation between Length of Chronnosome and Number of Chias- 

 mata. — In Mecostethus all chromosomes have one chiasma, whatever their 

 length. In other organisms the short chromosomes usually have one chiasma and 

 the longer ones more. In Fritillaria imperialis there are two clones, a and b, with 

 slightly different chiasma frequencies; in both, the short chromosomes, which are 

 probably fragments broken off normal chromosomes, may have less than one 

 chiasma per bivalent. 



(From Hearne and Huskins.) 



relation with length is not always of the same form. In Fritillaria there 

 is simple proportionality, but in most organisms there is a tendency for 

 one chiasma at least to be formed even in the shortest chromosomes. In 

 Mecostethus with localized pairing there is very rarely more than one 

 chiasma found whatever the length of the chromosome, though occa- 

 sional chiasmata are found at the distal ends of some chromosomes.^ 



The relation between chiasma formation and length led Darlington 

 to formulate the theory that the chromosomes forming a meiotic 

 bivalent are only held together by the occurrence of chiasmata between 



1 White 1936. 



