THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES 



99 



gland chromosomes allows an exact allocation of definite genes to defi- 

 nite positions in the chromosome ;^ and the fact that the chrom.osomes 

 are paired, enables one to draw detailed conclusions as to the sorts of 

 pairing to be expected in organisms with chromosomal abnormalities. 

 In salivary glands many chromonemata appear to be able to pair at a 

 point, while in zygotene pairing is only in twos (p. 69). Pairing of 





Fig, 46. Maps of the X Chromosome of D. melanogaster. — Above is the mitotic 

 metaphase chromosome with the position of some breaks occurring in trans- 

 locations, etc., and below it the mutation frequency map (both after Muller and 

 Painter) and the normal cross-over map. The cross-over map is also shown on a 

 larger scale and compared with the salivary gland chromosome map (after Muller, 

 Painter, and Mackensen). Notice how the genes appear concentrated at the 

 ends of the cross-over map. Sp spindle fibre attachment or centromere, Cc 

 chromocentre. 



salivary chromosomes must therefore be brought about by some forces 

 different to those concerned in normal meiotic pairing; they may be 

 related to the forces which produce somatic pairing (p. 74), which is 

 strong in Diptera. 



The appUcation of observations of salivary gland pairing to zygotene 

 pairing is probably justified; so far it has not led to any contradictions 

 but to extensions of principles which had already been deduced from 

 direct observation. The most important of these principles is that pair- 

 ing is strictiy between similar points of the chromosome. The pointwise 

 pairing may be complete even between a deficient chromosome and a 

 ^ Bridges 1935, 1938, Painter 1934, 1935. 



