CHAPTER XXI 



INHERITANCE IN MAN 



Detailed and accurate knowledge of variation and inheritance in man 

 would probably be of more practical worth than any other contribution 

 biology has made or can make to humanity; but for practical as well as 

 biological reasons, this is probably the most difficult and baffling of all 

 fields of biological research. We have seen that the main key to a knowl- 

 edge of genetics is experimental breeding — a method which is ruled out 

 in our attempt to understand human genetics. Both cytological and 

 statistical methods of investigation are most useful and efficient when 

 they can be combined with breeding experiments upon the same stock of 

 organisms. Moreover, human cells with their 48 chromosomes are much 

 more difficult to study cytologically than are the cells of Drosophila, 

 with their 8 chromosomes, or the cells of the other organisms from which 

 our knowledge of cytology is mainly derived. 



Most of our knowledge of human genetics comes from the scrutiny 

 and statistical study of human pedigrees and from comparison of the 

 modes of inheritance thus indicated with similar pedigrees in experi- 

 mental organisms. Some special handicaps to this procedure lie in the 

 following conditions: the small size of human families; the very slow 

 reproductive rate, with an average of about 3 or 3^ generations per 

 century; the markedly heterozygous constitution of most human stocks; 

 and the extremely numerous and complicated factors that make up the 

 environment of civilized man. 



Heredity versus environment in relation to man. Perhaps the great- 

 est difficulty of all lies in the fact that any individual man or woman 

 is the product of both heredity and environment and that it is extremely 

 difficult or impossible to know which of his or her individual qualities 

 are chiefly due to heredity and which to environment. We can some- 

 what clarify the question by an attempt to analyze, define, and dis- 

 tinguish between the two effects. 



Sir Francis Galton, who first attempted a careful analysis of the com- 

 plementary roles played by heredity and environment in man, distin- 

 guished between what he termed nature and nurture. By nature is meant 



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