168 THE CHROMOSOMES 



there is no explanation as to why the factors in a 

 group remain together in sections as explained on 

 page 66. If it is supposed that the chromosomes 

 break only once or twice, and that Hnkage represents 

 the holding together of the pieces, then one is forced 

 to assume that the breaking up is the same in both 

 members of a pair, yet entirely inconstant in different 

 cells; for otherwise the reunion of the fragments 

 would lead to duplication or loss of whole sections 

 of the chromosomes, and all order would soon be lost. 

 A large amount of data relating to sex linked char- 

 acters has shown that the sex chromosomes must 

 remain intact as often as they break apart, and even 

 when they break apart this takes place, as a rule, at 

 only one place. 



Other Theories of Crossing Over 



The ''reduphcation" theory of Bateson and 

 Punnett has been treated in another section (see 

 page 74) and the discussion need not be repeated 

 here. 



It has been claimed by Goldschmidt that, even 

 though the groups of factors are admitted to be 

 carried in the chromosomes and to undergo some 

 sort of interchange before reduction, it is not neces- 

 sary to assume that this interchange is of the nature 

 of a crossing over of the threads. He has postulated 

 that during resting stages the genes are set free 

 from the forces that hold them in their places in the 

 chromosomes, and that when they reassume their 

 places before division the two allelomorphs may 



