THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES 9I 



a chromosome and in different chromosomes (and according to the 

 methods adopted by different people for calculating it!).^ There is no 

 interference between cross-overs on different sides of the centromere 

 in the V-shaped autosomes of Drosophila. For a cytological discussion 

 of interference, see p. 125. By adding to the linkage or recombination 

 values the number of breakages which, because of double crossing-over, 

 do not result in recombination, we can obtain the total frequency of 

 cross-over between two factors. These values, which can only be 

 arrived at by calculation from the crude recombination data, are known 

 as cross-over values. A cross-over map represents the chromosome as 

 a straight Une on which the genes are placed at distances apart propor- 





Fig. 40. Diagram of Single (above) 

 and Double (below) Cross-overs. 



tional to the cross-over values. For this reason the cross-over value 

 between two genes is also known as the map-distance. 



The most complete cross-over maps are those of Bridges for D. 

 melanogaster^ but considerable numbers of factors have been discovered 

 and mapped in other Drosophila species. ^ The ten chromosomes of 

 maize have also been fairly well filled with genes^ and less complete 

 maps can be prepared for the sweet-pea,* locusts,^ etc. 



The Drosophila maps show a moderately even spacing of genes 

 throughout the chromosomes, but even here there are regions in which 

 the genes appear to be unduly concentrated. These regions are the 

 distal end of the X, that is the end farthest from the centromere, and 

 the middle of the large autosomes immediately in the neighbourhood of 

 the centromere, with less marked concentrations at the proximal end of 

 the X and the ends of the arm of the autosomes. (In the small IVth 

 chromosome, true cross-over is only doubtfully recorded, the few 

 recombinations that occur may be due to mutation.) The points of 

 crowding in the cross-over maps are much more marked in locusts. 



The crowding of genes at particular parts of the map is the same as 

 comparative lack of cross-over, and is a function of the position, since 



^ Cf. Anderson and Rhoades 193 1, Stevens 1936. 



^ Cf. Morgan 1926, D. I. S. brochures passim. 



' Emerson, Beadle and Frazer 1935. * Punnett 1927. ^ Nabours 1929. 



