LINKAGE 59 



Non-crossovers Crossovers 



White round Red bar White bar Red round 



28 per cent. 28 per cent. 22 per cent. 22 per cent. 



56 per cent. 44 per cent. 



Here a large amount of crossing over appears, about 

 44 per cent. In fact, so freely do the factors inter- 

 change that without sufficiently large and accurate 

 numbers the linkage might entirely escape detection. 



THE MECHANISM OF CROSSING OVER 



If it be admitted that the Mendelian factors are 

 carried by chromosomes it can not be denied that 

 interchange between homologous chromosomes must 

 occur, for sex linked factors cross over from each 

 other, and yet are known to be in the same pair of 

 chromosomes, since they all follow the X chromo- 

 some in its distribution. The evidence allows for no 

 other interpretation. But why should crossing over 

 take place so rarely between certain factors and so 

 often between others? We can make use here of 

 certain information in regard to the chromosomes 

 that gives a very simple answer to the question. In 

 the early germ cells, before the maturation period 

 begins, the chromosomes appear to be scattered in 

 the nuclei, and the homologous chromosomes in 

 many cases show no tendency to lie together, although 

 in some animals, e.g. in many flies, the members of a 

 pair are often found side by side. In this early period 

 the germ cells divide as do other cells and thereby 

 increase in numbers. But at the termination of this 



