DISPROOF OF A CERTAIN TYPE OF THEORIES 

 OF CROSSING OVER BETWEEN 

 CHROMOSOMES 1 



PROFESSOR H. S. JENNINGS 

 Johns Hopkins University 



Two types of relations have been proposed to account 

 for the facts of "crossing over" between pairs of char- 

 acters that follow the same pair of chromosomes. One is 

 a varying relation between the substances forming the 

 factors belonging to diverse pairs in the same chromo- 

 some; the other a varying relation between the two mem- 

 bers of the same pair, in the two paired chromosomes. 



The former type is represented by the "chiasmatype" 

 theory, held by Morgan and his associates, in which the 

 diverse relations are held to be, or depend upon, the ac- 

 tual diverse distances apart of the factors extended along 

 the linear chromosome. When the chromosome breaks, 

 for any cause, it is more likely to separate two factors far 

 apart than two close together; on this depends the vary- 

 ing cross-over ratios. 



The second type is that brought to notice recently by 

 Goldschmidt (1917), and commonly called the "variable 

 force" theory. It is conceived that the two members of 

 a given pair, as A and a, in the two paired chromosomes, 

 may be held or drawn to their places by a pair of varying 

 forces, which allow them to exchange places on the aver- 

 age in a certain proportion of cases ; while B and b are 

 held by a different pair of forces, which allows these two 

 to interchange in a different proportion of cases ; C and c 

 by a still different pair, etc. The result would be diverse 



iThis paper arose and took shape during discussions on theories of 

 crossing over in the Seminary on Genetics at the Johns Hopkins University. 



