CHAPTER XXXV 



HUMAN HEREDITY AS REVEALED BY PEDIGREES 



Weaknesses of the pedigree method. — The older type of pedigree 

 study involved the collecting of pedigrees made out by persons un- 

 trained in genetics and dealing with individuals in their own family 

 connections. Questionnaires were sent out to large numbers of indi- 

 viduals asking them to give data as to characteristics of their ancestors 

 and other relatives. Much of the material thus collected depended 

 upon the vague memories of individuals about the characteristics of 

 long-dead ancestors or of relatives in collateral lines. The low degree 

 of reliability of such data makes conclusions therefrom almost useless. 

 Considerably more reliable data were obtained by field workers who 

 actually observed individuals belonging to four, in extreme cases five, 

 generations, all living simultaneously. The reliability of such data 

 depends upon the training and scientific aptitude of the field worker. 

 Fortunately, there has developed in recent years a new profession, 

 that of eugenic field worker, and there are not a few very capable per- 

 sons belonging to this profession. Another weakness of the pedigree 

 method is that it affords no check on the actual parentage of offspring. 

 The mother is usually not in doubt, but there is often some question 

 as to the actual father. This is principally true in connection with 

 the study of pedigrees of feeble-minded and delinquent stocks and of 

 primitive peoples with moral codes somewhat less rigid than our own. 

 In other words, the genetic data acquired by this method are not scien- 

 tifically controlled. Still another weakness of this method is that it is 

 not always possible to distinguish between conditions that are heredi- 

 tary and those due to disease or poor environment. In spite of these 

 weaknesses, the pedigree method has brought to light a great deal of 

 valuable information about human heredity. So long as one retains a 

 properly critical attitude toward data and conclusions, no harm and 

 much good may accrue from a reveiw of the principal results of this 

 method of research. 



Two postulates underlying the study of the heredity of human 

 traits may be set down as follows: (i) When characters are inherited 

 according to the laws of Mendel, that is, when one of a pair of charac- 

 ters is dominant, or partly so, and the other recessive, and they segre- 



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