CONTINUOUS VARIATION 



variation, a discontinuity which permits the analysis of the system 

 by the nicncichan method. Genes hke these are often termed poly- 

 meric genes. All polygenes are therefore polymeric, but not all 

 polymeric genes are polygenes. 



Polygenic Systems 



Two questions immediately arise about polygenic systems. First 

 we see that such systems can explain continuous variation; but is 

 continuous variation in fact to be ascribed solely to their operation ? 

 Or to put it another way, we have been led to postulate systems of 

 genes which cannot be analysed by the mendelian method. How then 

 can we be sure that these genes are inherited in accordance with 

 mendelian principles i Secondly, since we cannot follow the genes 

 individually, we cannot describe the properties of these systems in 

 terms of individual segregation ratios and linkage values. How then 

 are we to understand and express and predict the behaviour of the 

 systems ? To the first of these questions we must now turn. The 

 second will be discussed in the next chapter. 



We have seen in the previous chapter that mendelian inheritance 

 reflects the fact that the genes concerned are carried by the chromo- 

 somes. The cytological study of meiosis shows that two properties 

 must mark this type of transmission. All the genes must show 

 segregation, and those which are borne in the same chromosome 

 must show linkage with one another. If, therefore, we can establish 

 that the determinants of heritable continuous variation show both 

 segregation and linkage, we caimot avoid the conclusion that they 

 are nuclear genes and that as such they will conform to mendelian 

 principles. 



Where a stock or line of a species has long been inbred we must 

 expect that its members will be homozygous for all or nearly all 

 their genes and that differences between them will be non-heritable. 

 Johannsen's experience confirmed this with beans. If, now, we cross 

 two such lines with one another, the F^ will be genetically as uniform 

 as its parents. Though it will be heterozygous for any genes in which 

 the parental lines differed, all the members of this generation will 

 be alike in their heterozygosity. Thus variation in the Fi will also 

 be non-heritable. But in the Fg the individuals will not all be alike 



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