Heredity — Genetics 713 



thetic understanding of human behavior, inheritances of diseases, and 

 temperaments. Human inheritance is also considered in the chapter, 

 Biology of Man. 



Methods of Studying Human Inheritance: The following methods 

 may be more or less successfully used in our study of human inheritance: 

 (1) A study of genealogic records collected by families, public institu- 

 tions, or scientific observers. (2) Selective mating which corresponds to 

 experimental crossing in lower organisms. Certainly many defectives 

 should never be permitted to pass these defects to their offspring. The 

 latter will be personally handicapped and will in turn transmit them to 

 their offspring. This is costly for human society at large. Undoubtedly, 

 a wise selection of a prospective husband or wife can never be regretted 

 from the standpoint of heredity. It is from studies of selected marriages 

 that much of our information of human heredity has been secured. (3) 

 Cytogenetic methods, in which a study of the cellular construction of 

 the heredity mechanism of human beings is made, which should prove 

 profitable. However, many obstacles prevent as much progress in this 

 direction as might be desired. 



Difficulties Encountered In Studying Human Heredity: The study of 

 human heredity is quite difficult and complicated. This is true because: 

 ( 1 ) The length of human life, especially the time required for individuals 

 to reach maturity, is quite long, so that a single observer, at best, can 

 see or study only three or four generations during his lifetime. This is 

 a handicap because one of the necessities for the successful study of any 

 heredity problem is the availability of a large number of successive gen- 

 erations. (2) Some individuals are reluctant to be studied. Frequently 

 inaccuracies are recorded by observers because the persons who volunteer 

 the information either willfully or unknowingly give the wrong informa- 

 tion or impressions. (3) There is a lack of scientific and accurate rec- 

 ords of human characters, if any record has been kept at all. Often, 

 only the defects or particularly outstanding characters are Hsted or 

 recorded. Certain individuals will give inaccurate information to "cover 

 something up," or will give a type of information which will tend to 

 make their family appear much better than it really is. Unless the ob- 

 server can collect all his data personally, this is a severe handicap. (4) 

 Many phases of human heredity are much more complicated than they 

 are in lower organisms. With our present limited knowledge of heredity 

 in general, we are unable to make great progress in the field of compli- 

 cated human heredity. (5) The lack of definite description and defini- 

 tion of human traits, such as mentahty, insanity, musical abilities, genius, 



