CHAPTER 3 



CONTINUOUS VARIATION 



The Limitations oj Mendelism The Specification oj Continuous Variation 



Heritable and Non-heritable Diferences Cumulative Effects of Genes 



Polygenic Systems Linkage oj Polygenes 



Mendel's bequest to genetics was twofold. He gave us his 

 principles of inheritance, and at the same time he gave us the 

 experimental method by which he had been able to establish those 

 principles and by which they could be tested and extended. This 

 method, as we saw in the last chapter, has enabled us to show that 

 Mendel's principles reflect the properties of inheritance through the 

 nucleus. Wherever the hereditary determiners of variation are 

 carried by the chromosomes we must therefore expect these 

 principles to apply. 



We cannot, however, expect the experimental method to be 

 equally widely applicable. This depends for its success, as Mendel 

 himself clearly realized, on the differences under investigation (tall 

 V. short, round v. wrinkled, etc.) being so large compared with 

 any residual or extraneous variation in the character, that individuals 

 could be assigned unambiguously to one or other of the alternative 

 classes. Thus Mendel's short peas ranged in height from 9 inches to 

 18 inches, and his tall parents from 6 feet to 7 feet. The F^'s of tall 

 and short even exceeded the tall parent with heights up to j\ feet. 

 But in spite of this variation in both tails and shorts, there was 

 never any doubt that a plant, whether in the parental, F^,' Fg, or any 

 other generation, was either the one or the other. The variation 

 was in fact what came to be called discontinuous: all below the 

 discontinuity in the distribution of heights were shorts and all 

 above it, tails. 



The residual variation, within the tails themselves and equally of 

 course within the shorts, was evidently not of this kind. There was 

 presumably a smooth, or at least an apparently smooth, gradation 

 in height between the shortest and tallest tails and between the 

 shortest and tallest shorts. The variation within these classes was 



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