HEREDITY 



599 



Genetic 

 components 



Trait 



Environmental 

 components 



-<r 



Figure 424. Diagram to show the relative influences (components) of heredity and environ- 

 ment in the development of traits. The black symbol lettered "H" illustrates a hypothetical 

 trait, having about equal genetic and environmental components. The other symbols represent 

 different traits (phenotypes); the length of the solid and dotted lines shows the relative im- 

 portance of the genetic and environmental components. No trait could occur at either extreme 

 since all traits possessed by an organism are influenced by both genetic and environmental fac- 

 tors. In many cases, one cannot measure the effects of one or the other, but in this diagram 

 traits have been selected where the relative influences of environment and heredity are known. 

 "A" symbolizes the phenotype albinism (absence of pigment), a trait which develops because of 

 the presence of an allelic pair of recessive genes. Here the environment provides only the minimal 

 essentials. "Ci" represents a cancer of the eye, retinoblastoma, a trait which is largely influenced 

 by a single dominant gene. The environment, however, must play a part in its development. 

 "C," symbolizes another malignancy, breast cancer, a trait for which there is statistical evidence 

 of genetic factors, but there are also several important environmental factors known to be operat- 

 ing in the development of this trait. "W" represents the trait "writing," a behavior charac- 

 teristic for which most of mankind possesses the necessary physical equipment. However, such 

 a behavioral trait also requires training, an environmental component lacking in many cultures. 

 An additional aspect of heredo-environmental interactions, not illustrated by the diagram, is 

 the fact that the same phenotype may result from both a "weaker" and a "stronger" genetic 

 component if the "weaker" is augmented by a "stronger" environmental component; in other 

 words, the same genetic component may result in different phenotypes if different environments 

 are provided. The best evidence for this may be found in the differences observed in "identical" 

 twins in different environments. 



hereditary feeble-mindedness have a much 

 higher birth rate than normal people. Fig- 

 ure 425 shows how such an unfortunate 

 differential birth rate works. It has been 

 estimated that there are about three million 

 feeble-minded persons in the United States. 

 There have been many inaccurate state- 

 ments about how little feeble-mindedness 

 would be reduced in a generation if all 

 feeble-minded persons of a previous genera- 

 tion had not reproduced. Most of the incor- 



rect figures given have assumed that the 

 feeble-minded marry at random as fruit 

 flies mate in a bottle. It has been proved 

 that this is a false assumption and that the 

 feeble-minded tend to marry feeble-minded. 

 It is impossible to determine exactly how 

 much reduction in the feeble-minded would 

 be effected in a generation if they did not 

 procreate, but something would be accom- 

 plished. 



Eugenics favors measures that may assist 



