A CRITICISM OF THE HYPOTHESIS OF LINKAGE 

 AND CROSSING OVER. 



By a. H. trow, D.Sc, F.L.S. 



The publication of The Mechanism of Mendelian Heredity by 

 Morgan, Sturtevant, Muller, and Bridges, marks a definite stage in the 

 devel(3pinent of the hypothesis of linkage and crossing over. The 

 authors' faith in this hypothesis has evidently become so strong that 

 they are not unlikely to infect others with their belief irrespective of 

 any i-eal demonstration of its validity. It seems therefore desirable 

 that the hypothesis shoulil be subjected to independent criticism. 

 Such criticism is really rendered necessary by the fact that although 

 the authors devote much time and space in their book to the elucidation 

 of the simple Mendelian ratios, they give no clear coherent account of 

 their mode of explanation of the more complex and troublesome ratios 

 •which students of genetics cla.ssify under the headings coupling, re- 

 pulsion, reduplication, and crossing over. 



Drosophila ampelophila , the type which these authors have mainly 

 investigated, appears to be an organism admirably suited to genetic 

 analysis. Already more than a huntlred factors have been isolated 

 and their relationships studied in some detail. The authors may be 

 congratulated on their choice of such excellent material, and on the 

 skill and industry with which they are exploiting it. One peculiarity 

 of the organism deserves special mention — the *• or n number of 

 chromosomes is four, and the set is remarkable in that one of them 

 is spherical in form, the remaining three being rod-like. 



In the study of the hundred factors already recognised in this 

 oi-ganism, many deviations from the normal gametic and zygotic ratios 

 have been recognised, and it is these which have apparently led the 

 authors (who reject the reduplication hypothesis) to elaborate their 

 alternative hypothesis of linkage and crossing over. 



