Chromosomes and the Theory of Heredity^ 



By C. D. DARLiiVGTON, F.R.S. 



Botany School, Oxford University, Oxford, England 



MORGAN'S DISCOVERY 



The crisis of the struggle in the scientific world over the chromosome 

 theory of heredity was reached in the 1920's when T. H. Morgan's 

 views became the subject of dispute. Morgan, with his collaborators 

 at Columbia University, had carried out breeding experiments with the 

 fiy Drosophila [1].^ He claimed that by these experiments he could 

 show that heredity, as long suspected, was indeed carried entirely by 

 the chromosomes. It followed that these minute bodies in the cell 

 nucleus were to be held responsible for the whole character of every 

 living thing, plant or animal, man or microbe; and the course of evo- 

 lution from the beginning had been determmed by changes in these 

 chromosomes. It was a complete scheme of determinism on Omar 

 Khayyam lines. 



This theory aroused misgiving and contradiction in many countries, 

 especially among older men who might know the fly but certainly did 

 not know its chromosomes. Before we look at their arguments, let 

 us see what Morgan, and Mendel before him, had done. 



Mendel had found that if he crossed two races of peas differing 

 in two respects, as it might be AB X ab^ the hybrid gave germ cells of 

 four kinds in equal numbers: AB^ Ah^ aB, and ab. Free assortment, 

 random recombination, independent segregation were the explanations 

 given for this behavior. Assortment of what? Assortment of certain 

 "elements" carried in all cells and passed to the germ cells. Morgan, 

 however, beginning in 1910, found that in the fly often less than half 

 the germ cells were of the new types. Ah and aB. The proportion was 

 characteristic of the particular pair of elements. Assortment was not 

 free. The elements were linked; and, if the hybrid fly happened to 

 be a male, the linkage was complete : the elements were held together 

 in one block. 



* Substance of a Royal Society Tercentenary Lecture delivered on July 20, 1960. Re- 

 printed by permission from Nature, vol. 187, No. 4741, pp. 892-895, Sept. 10, 1960. 

 ' Numbers in brackets refer to list of references at end of article. 



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