THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES 



97 



in a definite piece of the map, not a collection which is scattered hap- 

 hazard, as might be expected if the map did not correspond with 

 material reality. Further, cj^ological investigation often reveals an 

 actual chromatin fragment in an unusual position, and this fragment 

 can be correlated with that part of the map which genetic analysis 

 shows to be affected. A chromosome map can thus be prepared, in 

 which the position of the genes on the actual chromosome is marked. 

 The maps prepared by study of translocations in metaphase chromo- 

 somes in Drosophila show considerable differences from the cross-over 

 maps.^ In particular, regions where the genes are crowded together in 



al dp 



D th St pcu 



Fig. 45. Cross Over and Mitotic Metaphase Maps of the 2nd and 3rd 



chromosomes of D. melanogaster. 



(From Timofeef Ressovsky, after Muller, Painter, and Dobzhansky.) 



the cross-over map are found to occupy a considerably greater propor- 

 tion of the metaphase chromosome map. In Drosophila melanogaster 

 the crowded regions are mostly near the centromeres (middle of II and 

 III, left end of X). The crowding here is partly explained by the fact 

 that these regions of the chromosome are occupied by "inert" or 

 heterochromatic material, in which no genes, or very few, are known 

 and which are therefore not represented on the cross-over map. But 

 even if we make allowance for this, and add to the cross-over map a 

 section corresponding to the inert region of the metaphase chromo- 

 somes, we still find that there is some crowding of genes in the regions 

 just outside the heterochromatic parts. This represents a lower fre- 

 quency of cross-over per imit of material length than in other parts. 

 The scale on which a cross-over unit represents material length is thus 

 not constant. The greatest amount of crossing-over per unit of material 

 length is in the middle of the X chromosome and in the middle of the 

 autosomes, that is to say, it is as far as possible away from the 

 ^ Dobzhansky 1929, 19306, Painter and Muller 1929. 



