282 CHROMOSOMES IN HEREDITY : MECHANICAL 



No chromosome has been definitely measured and found to have 

 a length of less than 50 units. If the pairing of the small fourth 

 chromosome in Drosophila melanogaster is due to chiasma formation 



6/t 3/t 3/0 4/0 2/2 2/0 3/0 I/O 2/t 



24/ 



4/2 4/2 3/2 3/2 2/. 1/, 2/, 2/2 2/, 



3/2 ^A 2/, 2/, I/, 2/, 2/, I/, I/. 2/ 



Fig. 93. — Chiasmata in Zea Mays. Above, late diplotene ; 8 

 bivalents and one ring of four, 8-9 interchange. Note differential 

 condensation. Middle, same plant at first metaphase, centromere 

 obliterated by pressure in some bivalents. Below, double trisomic 

 plant with reduced chiasma frequency. X 2000 (D., i934' <^/- 

 Fig. 143)- 



the chiasmata would need to be strictly localised and map distances 

 would not have the same relation to length as in the other 

 chromosomes. Evidence of this condition has been found by 

 Mather (1936 h). Two factors close together in this chromosome 



