CONSTITUTION OF THE CHROMOSOMES 



149 



a way as to separate, into different chromosomes and differ- 

 ent individuals, two genes such as club and miniature in 

 figure 33, II, then all the genes on one side of the break 

 must go together to one individual, while all those on the 

 other side of the break must go together to the other indi- 

 vidual. Thus, in case of a break between club and vermilion 

 (figure 33), the genes for yellow, white, abnormal and bifid 

 would go with club to one individual, while the genes for 

 bar, rudimentary and miniature would go with vermilion 



Figure 34. Diagram to illustrate crossing-over. In A, the paternal 

 (p) and maternal (m) chains of genes. In B (a later stage), a part 

 of the chain p (white) has become united with a part of the chain 

 m (black) ; the two have exchanged part of their genes. 



to the other individual. Crossing-over would therefore not 

 be confined to single genes, but would occur in blocks; and 

 the genes that go together could be predicted from a knowl- 

 edge of the exchange ratios of the genes. 



This furnishes a crucial test of the theory, for if the 

 genes are not in a linear series, this relation is not to be 

 expected. Examination shows that the requirement set forth 

 in the preceding paragraph is indeed fulfilled. Crossing-over 

 actually does occur in blocks of genes, those shown to be 

 near together in the linear diagram (figure 33) going to- 

 gether to one individual; while a block of those that are 

 together in a distant part of the series go to the other in- 

 dividual. Figure 34 is a diagram showing what occurs at 

 crossing-over. 



As to the intimate processes that occur in the chromo- 



