THE UNION OF GENETICS AND BIOMETRY 



the effective factors are not necessarily fixed in the course of an 

 experiment and may increase in number. Their history will depend 

 on the circumstances of localization of crossing-over which we have 

 already noticed, and also, as we shall see later, on various restrictions 

 of crossing-over which arise in hybrids. 



When the individual genes have large effects unlike one another, 

 we can follow them and count them as individuals by the mcndclian 

 method. A single recombination between them, however many tests 

 must be made before it is found, is sufficient to show that two 

 distinct genes are at work. Such a unique recombination can be 

 detected without ambiguity by the mendelian technique when the 

 genes have different effects on the phenotype. But the members of 

 a polygenic system cannot be followed individually in inheritance, 

 nor their recombinations identified as individual events. In counting 

 the units of polygenic inheritance we are therefore forced to use 

 a coarser criterion, that of the occurrence of a particular recom- 

 bination frequency, usually 50 per cent. The effective factors whose 

 number we estimate are thus, in the general case, not ultimate 

 polygenes. Indeed we can have no certainty that any unit of 

 polygenic inheritance, which we may find and whose properties 

 we may determine, is an ultimate unit. 



We are debarred from studying polygenes as individuals by the 

 inherent limitations of genetical method. Rather we must follow 

 them as they are organized into effective factors. These units are 

 not final in the way that individual major genes are. They can be 

 broken down into smaller parts by recombination and they can 

 presumably be synthesized by bringing together their parts through 

 recombination. Their properties must depend on the way the genes 

 are put together to make the factor as much as on the genes them- 

 selves. 



The Union of Genetics and Biometry 



Biometrical genetics is built upon the foundation of mendelism. 

 It could exist in no other way, and the relative failure of Galton and 

 the early biometricians to achieve an understanding of inlieritance 

 was due to the lack of the foundation which Mendel, by an entirely 

 different technique, was able to supply. There is thus no conflict 

 between biometrics and mendelism. On the contrary, biometrical 



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