CHAPTER XII 



THE CHROMOSOME THEORY 



Of the more recent developments in Mendelism, 

 none have excited so much interest or been so 

 productive in results as those associated with the 

 names of Morgan and his colleagues in America. 

 With some of these we have already dealt in con- 

 nection with sex. The little fruit fly, Drosophila 

 ampeiophila, with which they worked, is the most 

 favourable material yet found for unravelling the 

 problems of heredity. It already offers more than 

 lOO factors which appear to exhibit Mendelian 

 segregation : its chromosomes are distinct and few 

 in number : it is both hardy and prolific : it is 

 easily dealt with in the laboratory as it takes up 

 little space, and it is cheap to work with, since 

 thousands may be reared at the cost of a few rotten 

 bananas. During the past ten years these advantages 

 have been fully exploited by the American workers, 

 and their results have led them to formulate what 

 is known as the chromosome theory of heredity. 

 According to this theory the chromosomes form the 

 material basis of heredity. The various factors 

 upon which the manifestation of characters depend 



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