D. S. Falconer, Ph.D. 



QUANTITATIVE INHERITANCE 



A quantitative character is any attribute for which individual differences do not 

 divide the individuals into qualitatively distinct classes. Antigenic differences, for 

 example, separate individuals into qualitatively distinct classes and are therefore not 

 quantitative characters. But an antibody titer, which varies continuously from one 

 individual to another, is a quantitative character. The inheritance of quantitative 

 characters is generally under the control of many genes (that is, it is polygenic), but 

 this is not necessarily so. The essential feature is that the segregation of the genes, 

 whether few or many, is not manifest in phenotypic discontinuity. This feature of 

 quantitative characters precludes the application of the ordinary methods of genetic 

 analysis used for the study of single-gene effects. Special methods are therefore re- 

 quired, and at the same time the questions that can be answered are different in nature. 

 In general, the genetic properties of a quantitative character that can be investigated 

 are those arising from the simultaneous action of many genes. It will not be possible 

 to explain fully the theoretic nature of these properties here; nor will it be possible to 

 describe all the methods available for their study. So this discussion will be confined 

 to a description of the simpler methods which may be of use to those for whom genetics 

 is a secondary rather than a primary interest. Explanations of many of the points 

 which cannot be fully explained here will be found in the treatise by Falconer. 335 



The first and simplest question to be asked about any quantitative character is: 

 are there any genetic differences ? To answer this question it is only necessary to com- 

 pare different strains maintained in the same place and under the same conditions. 

 If strains differ in their mean values of the character this proves the existence of genetic 

 differences. So many characters are known to exhibit strain differences that the exist- 

 ence of genetic differences in any character can almost be taken for granted. The 



193 



