Biology of Man 523 



in the scientific study of human inheritance are: (1) the impossibility 

 of securing complete and reliable data over a sufficient number of gen- 

 erations, (2) the great difficulty of securing reliable and complete data 

 for all members of the family being studied, (3) the impossibility of the 

 scientific observer to collect all the data firsthand and the frequent un- 

 reliability of data supplied by others who are insufficiently trained in 

 heredity, (4) the small size of many families which does not always give 

 a complete picture of the particular inheritance being studied, (5) the 

 inability to cross or breed experimentally as has been done so profitably 

 in lower organisms, and (6) the great length of time required for, let 

 us say, three consecutive generations to display what they have inherited. 

 In spite of these difficulties, much progress has been, and is being, made. 

 This explains why it is unknown whether certain traits are inherited and 

 why there is still a difference of opinion as to the exact method of in- 

 heritance of certain human traits. 



It is known that there is a continuity of germ plasm (Figs. 350 and 

 351) connecting the individuals of all generations, and through which 

 traits are transmitted to future generations. Body cells (somatic cells) 

 with their traits develop from these germ cells with their germ plasm. 

 The development of human traits depends upon the presence of specific 

 genes within the chromatin materials of the germ plasm and the develop- 

 ment of these genes in the proper environments as the body grows. Pos- 

 sibly the cytoplasm of these cells plays a greater role in inheritance than 

 we have surmised in the past. Human heredity studies have usually 

 concerned themselves with the more obvious traits (such as eye color, 

 hair color, hair consistency, skin color), or certain abnormal traits (such 

 as webbed toes, psychosis, etc. ) , but in all probability the more common 

 traits (such as stomach, liver, intestines, etc) also have a structural and 

 functional inheritance. Is it not possible that there is a typical Smith- 

 family- type of stomach or a Jones-faniily-type of intestine? Because 

 they have been considered to be of secondary importance such traits have 

 not been generally studied. Infectious diseases are not believed to be 

 inherited, as such, because the transmission of the causal agents for 

 such diseases in the germ plasm would seriously affect or prevent the 

 formation of the offspring. However, predisposing tendencies for cer- 

 tain diseases may be transmitted. Certain conditions have erroneously 

 been thought to be inherited, when, as a matter of fact, they have been 

 acquired by the offspring before or after birth. 



The average person is interested primarily in whether a certain hu- 

 man trait is inherited and how. Many human traits are considered in 



