So?ne Characters in Human Beings 45 



in the deposition of large quantities of pigment in the tissues, 

 bones, and teeth, and a red color in the urine. A dominant gene 

 seems to be the cause of a general allergic tendency which is 

 expressed in a great many forms. Some individuals possessing 

 this gene have hay fever, some have eczema, some have hives, 

 some have asthma, and others exhibit still other forms of hyper- 

 sensitiveness. 



The inheritance of psychological traits is, as a rule, more diffi- 

 cult to analyze than the inheritance of physical or of many 

 physiological characters. The most difficult of all is general 

 intelligence, a trait that is not easy to define or to measure. The 

 differences in intelligence between various individuals are not 

 clear-cut and there is a wide range of such differences, with very 

 superior persons at one extreme and very inferior ones at the 

 other and a continuous series of gradations between them. The 

 genetic situation is complicated for apparently a large number 

 of genes is involved in determining intelligence, and they may 

 interact in a very complicated fashion. It is a rather generally 

 accepted view among geneticists that the upper limits of a 

 person's intelligence are determined by his genotype, but how 

 nearly any individual ever reaches his upper limits depends 

 upon a great many factors such as training and other environ- 

 mental conditions. 



Certain grades of insanity are more readily susceptible of 

 genetic analysis partly because they are more readily recognized. 

 Dementia praecox apparently results from the interaction of 

 several recessive genes and is a condition in which a person 

 gradually withdraws into himself and lives in a dream world. 

 The manic-depressive type of insanity is also complex in its 

 inheritance and results from the interaction of several genes, 

 some of which appear to be dominants. A manic-depressive 

 has alternate periods of great elation and extreme depression. 

 These two types of insanity are hard to analyze genetically, but 

 that they can be identified simplifies the problem considerably. 

 Before they were recognized as distinct conditions and when 

 all types were lumped together under the term ''insanity," it was 

 impossible to make a genetic analysis. 



Certain other cases of low-grade mentality have been studied 

 sufficiently for at least part of the hereditary cause of the condi- 

 tion to be known. Huntington's chorea is known to be due to a 



