5IO OrganisiJis Are Products of 



Fig. 458 An albino child. Do you know why 

 he keeps his eyelids half shut when he is in 

 strong sunlight? How does this character act 

 in heredity? Was either parent necessarily an 

 albino? (acme) 



by two or several pairs of genes work- 

 ing together. Besides, many of them are 

 clearly affected by the environment, and 

 it is difficult for us to arrive at sensible 

 conclusions when we do not know how 

 much influence the environment has had 

 in each case. 



The inheritance of disease. Diseases 

 caused by bacteria or other microorgan- 

 isms, such as tuberculosis for example, 

 are caused by infection; they are not in- 

 herited. Yet it is possible that suscepti- 

 bility to a disease may be inherited. Sus- 

 ceptibility may depend in some way on 

 bodily characteristics, and thus be trans- 

 mitted from one generation to the next. 



Very little is known about the heredity 

 of organic diseases — diseases due to the 

 abnormal structure or working of some 



Heredity and Environment unit ix 



organ. But there is one unusual disease 

 of the nervous system (Huntington's 

 chorea) which is caused by a dominant 

 gene and is inherited in the Mendelian 

 fashion. 



Mental traits. Strictly speaking, "men- 

 tal" traits are also physical, because they 

 seem to depend on the structure of the 

 nervous system and on the ductless 

 glands. For convenience, however, we 

 often separate the two. Intelligence, per- 

 sonality, and behavior traits may be 

 called mental. 



What is intelligence? It is quite differ- 

 ent from knowledge. It is the ability to 

 obtain knowledge and to apply it in solv- 

 ing problems. We know that it varies in 

 people. As you know, school children 

 are often given "intelligence tests." These 

 tests contain some questions that require 

 certain kinds of knowledge and others 

 that test the ability to apply this knowl- 

 edge. The score that is made on the test 

 is compared with the age of the child. If 

 the score of a fourteen year old boy ex- 

 actly matches the score accepted as com- 

 mon for his age, his intelligence quotient 

 or I.Q. is said to be 100. If his score is 

 higher, his I.Q. is said to be higher. Fig- 

 ure 459 shows the graph or curve ob- 

 tained when the I.Q.'s of many children 

 are tabulated. 



Intelligence tests as they are given in 

 schools often result in inaccurate I.Q.'s. 

 For one thing, since the test is written 

 in English a child who is not familiar 

 enough with the language is at a disad- 

 .vantage. However, although intelligence 

 quotients are often inaccurate, testing is 

 likely to show up the highly intelligent 

 and those of very low intelligence, 

 known as feeble-minded. 



