HEREDITY 1 99 



families in which both parents were defective produced 470 feeble-minded 

 children but only 6 who were normal. When only one parent was defec- 

 tive the proportion was 193. to 144. Other matings, as well as gradations 

 from idiots to morons, seems to show that the common type of feeble- 

 mindedness is controlled by a small set of multiple genes which also are 

 recessive. 



Some mild types of epilepsy are allergic, being caused by such foods as 

 wheat; others are results of accidents or infections, and so cannot be in- 

 herited. But at least one tenth of all epilepsy is a distinct hereditary char- 

 acter, apparently being determined by multiple genes. Insanity, too, is a 

 hodgepodge; a variety of mental ailments that may or may not be related. 

 Some types are caused by accidents or genes, others by worry or extreme 

 unhappiness. 



MENTAL INHERITANCE IS NOT SIMPLE 



Family superiority is not traced to a single ancestor. One good parent 

 may produce a sporadic genius, but it takes two superior parents in gen- 

 eration after generation to keep a whole family notably above par. Con- 

 versely one bad ancestor is not enough to explain a defective family that 

 lasts and proliferates for generation after generation. We have no means 

 of knowing how much superior ability is helped by wealth and family 

 training; how effectively genius as well as mere normalcy can be swamped 

 by lack of money, broken homes or other demoralizing conditions. 



>> s 't ^vv_. 



THE STUDY OF HUMAN HEREDITY * 

 LAURENCE H. SNYDER 



One of the most interesting biological developments of the past decade 

 has been the increasing realization of the importance of a knowledge of 

 human heredity in everyday life. Of course a certain respect has been paid 

 to heredity for a long time. The considerations given matters of birth, 

 family background and race testify to this fact. It is only recently, how- 

 ever, that we have had any exact knowledge of the transmission of factors 

 for diverse characteristics from generation to generation in human beings. 



When the laws of heredity were discovered, tested and finally under- 

 stood in experimental plants and animals, it was inevitable that the atten- 

 tion of the geneticist should be drawn to the study of similar phenomena 

 in man. Gradually a body of knowledge on the genetics of man has been 

 built up, and, as always happens when sufficient basic facts are accumulated, 

 a series of practical applications has appeared. 



* Reprinted by permission of the Scientific Monthly, American Association for the 

 Advancement or Science. Copyright 1940. 



