Some Characters in Human Beings 37 



interesting gene that affects the body structure of mice. The 

 dominant gene, Sd, when heterozygous, produces either short 

 or no tails, often shortened or crooked spines, abnormalities of 

 the kidneys, and a generally lowered vitality (Fig. 13). The 

 homozygotes are completely tailless, have spines divided by a 

 cleft into two parts, have no kidneys or external genitalia, and 

 die shortly after birth. This whole complex of characters be- 

 haves as a unit. 



Some Characters in Human Beings 



For several reasons it is far more difficult to study the genetics 

 of human beings than inheritance in plants or in other animals. 

 The technique which is widely used in studying the way the 

 genes of plants and other animals are distributed among indi- 

 viduals, long known as the pedigree culture method, cannot be 

 applied to man because of the social nature of human beings. 

 Also, it is far more difficult to control or at least to analyze 

 the environmental conditions under which human beings de- 

 velop than those of other organisms, and unless the environment 

 can be eliminated as a variable, our genetic results are always 

 open to criticism. Many of the characters studied in other forms 

 are unit characters in which one gene alone is mainly responsible 

 for the development of a character; in human beings many of 

 the characters which have been studied appear to be caused by 

 the interaction of a number of different genes and are therefore 

 much more difiicult to analyze. Another complicating factor is 

 that in plants or other animals we can often deal with genes 

 having high penetrance, whereas in human beings many genes 

 seem to have reduced penetrance. Although several hundred 

 characters in human beings have been observed and studied, our 

 knowledge is satisfactory for only a small percentage. 



Studies in the heredity of human traits are complicated also 

 by the fact that in some instances two or more genes in dif- 

 ferent chromosomes may independently produce the same char- 

 acter, or characters that appear the same unless they are care- 

 fully studied. In human beings, some of our supposed traits, 

 especially some of the psychological ones, are themselves poorly 

 understood, and until the characters are recognized and distin- 

 guished, the genes that are active in their production cannot be 

 identified. Insanity illustrates the last point. Some years ago, 



