9 



Nonchromosomal genes 



Are Mendelian genes the sole hereditary determinants of the organism? 

 This question was first posed in the period 1900-1910, following the 

 rediscovery of Mendelian inheritance in 1900. Because the precision and 

 simplicity of unit factor segregation seemed so much at variance with 

 the obvious complexity of biological systems, many investigators ac- 

 cepted the evidence of Mendelian inheritance with great reluctance. 

 Some felt that the described instances were exceptional, that the attempt 

 to generalize "Mendelian laws" was pushing the data too far, and that 

 other mechanisms of heredity must also exist. 



Most impressive was the skepticism of T. H. Morgan, expressed just 

 a short time before he left his embryological research for genetics. In an 

 address to the American Breeders Association in 1909, he said: 



In the modern interpretation of Mendelism, facts are being transformed into 

 factors at a rapid rate. If one factor will not explain the facts, then two are 

 invoked; if two prove insufficient, three will sometimes work. The superior 

 jugglery sometimes necessary to account for the results may blind us, if taken 

 too naively, to the common-place that the results are often so excellently ex- 

 plained because the explanation was invented to explain them. 



Within a year Morgan began to work with Drosophila. and soon a 

 wealth of spontaneous mutants were isolated, providing magnificent 

 material for demonstrating the generality of Mendelian segregation and 

 leading to Morgan s formulation of the chromosomal theory of heredity. 

 The wide range of phenotypcs altered by mutation, and the rarity of 

 aberrant segregation, led many investigators to conclude that chromo- 

 somal factors could well account for the total heredity of the cell. 



However, also in 1909, the first paper on non- Mendelian heredity was 

 published. Its author, the German, Carl Correns, was one of the three 

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