CHAPTER IX 



THE STATISTICAL STUDY OF HEREDITY 



IN the preceding chapters we have considered the re- 

 sults obtained by the experimental method of studying 

 inheritance, according to which we observe the occur- 

 rence or non-occurrence of certain definite characters 

 through several generations in which the matings are 

 controlled. It will have become clear that there are 

 only three possibilities with regard to any particular 

 character viz. the two pure types and the hybrid. 

 Apart from the relatively unimportant matter of 

 dominance, we cannot in any sense speak of a degree 

 of resemblance between parent and offspring, provided 

 that the character in question is determined by one 

 factor pair. There are no degrees of inheritance. An 

 individual must either possess a particular factor pure 

 or hybrid, or lack it. 



It was formerly usual to speak of several different 

 kinds of inheritance, of blended, particulate inheritance, 

 and the like. But, as before mentioned, it seems un- 

 necessary to assume several fundamentally different 

 modes of inheritance. It seems probable that Mendelian 

 or alternative inheritance is the general rule. 



Nevertheless, it is possible to proceed to study 

 heredity by determining the average degree of resem- 

 blance between parent and offspring. 



In order to get a satisfactory illustration of the 

 method used, it will be best to consider some character 

 showing normal quantitative variability. One of the 

 most fully investigated cases is that of stature in man, 

 before mentioned. 



Variations in stature appear to be due to many 

 different causes, no one of which has, by itself, any very 



