LINKAGE RELATIONS IN MEN DELI SM 



109 



crossing-over occurs in 22.6 per cent, of cases. Accordingly the gametes 

 are formed in the ratio: 



38.7 per cent. (CW) : 38.7 per cent, (cw) : 11.3 per cent. (Cw) : 11.3 per cent. (cW) 

 77.4 per cent, non-cross-overs 22.6 per cent, cross-overs. 



It follows, therefore, that linkage may be interpreted as due to as- 

 sociation of factors within the same chromosomes and that crossing-over 

 or breaking apart of linked factors may be regarded as a consequence of 



FIG. 51. Diagrammatic representation of crossing-over and results. At the left, the 

 two original chromosomes. In the middle, the twisted condition of the chromosomes, in 

 synapsis and their subsequent separation. At the right, the four types of chromosomes 

 which result and their proportions. 



FIG. 52. Diagrammatic representation of crossing-over and its results when the factors 

 enter in the opposite combination from that shown in Fig. 51. 



chromatin exchange between homologous chromosomes during synapsis. 

 The factors may be thought of as the purely passive objects with which 

 the chromosome mechanism deals, they are linked together because they 

 are borne in the same chromosome, they show breaks in linkage in a 

 certain percentage of cases because in synapsis breaks occur between the 

 loci which they occupy in the chromosome such that new combinations of 

 the factors are formed. The chromosome relations are the same even 

 when chromatin interchange results in no new combinations of factors, 



