290 



CHROMOSOMES IN HEREDITY: MECHANICAL 



Crossing-over does not occur at meiosis between the autosomes 

 in male Drosophila, although it may occur at mitosis as a special 

 form of primary structural change (Ch. XII). This difference is 

 paralleled by the observed behaviour of the chromosomes (D., 

 1934; Dobzhansky, 1934, Fig. 116). The autosomes lie parallel to 

 one another, in pairs, with all four chromatids equidistant and 



h/'M.'s' 



345 6 7 8 910 



Chiasmata in Vicia and Fritillaria. 



Fig. 96. — Chiasma frequencies in the bivalents of Fritillaria Meleagris, 

 F. imperialis (various clones), and Vicia Faba {" m " and " M " 

 types of chromosome), showing decreasing interference values 

 with the higher mean values. (From D., 1931 c.) 



associated at the centromeres only, until metaphase, when the 

 centromeres separate. Pairing therefore takes place by an anoma- 

 lous mechanism without the apparent formation of chiasmata. Of 

 course crossing-over might have taken place without showing in 

 the production of chiasmata since chiasmata require also a special 

 association in pairs and a weaker secondary attraction. We can 

 merely say that the cytological and genetical observations agree in 



