510 - Heredity and Evolution 



factors from the manifold influences of the 

 child's environment. The best evidence re- 

 garding the inheritance of mental traits 

 comes from the study of identical twins, in 

 whom there is an exact duplication of all 

 genetic factors. Such twins, even if they have 

 grown up in separate and different environ- 

 ments, always show a very high correlation 

 of intelligence, as measured by several of the 

 available kinds of "intelligence tests." These 

 measurements are admittedly arbitrary, but 

 they do provide strong evidence that an indi- 

 vidual's capacity to learn is largely deter- 

 mined by genetic factors. Other traits, such 

 as are commonly designated as "personality" 

 and "character," arc less highly correlated in 

 given pairs of twins. This may merely mean 

 that the measurements of personality and 

 character are not very reliable; or more prob- 

 ably, it indicates that personality character- 

 istics are very susceptible to the modifying 

 influence of environmental experience. 



It has often been noted that "leeble-mind- 

 edness" and a few forms of insanity show a 

 marked tendency to "run in families"; and 

 the same is true for supernormal talents of 

 one sort or another. But closely related indi- 

 viduals tend to be exposed to the same envi- 

 ronment, especially during the formative 

 period of childhood, and the potency of the 

 environment cannot be ruled out under these 

 circumstances. On the basis of our present 

 knowledge, it is possible to say, however, 

 that the mental as well as the physical traits 

 of the individual are determined in highly 

 significant degree by genetic factors; but the 

 details — as to what and how many genes are 

 involved, and as to how the action of these 

 genes may be influenced by disease, malnu- 

 trition, education, economic welfare, and 

 other environmental factors — have not yet 

 been brought into focus very clearly. 



The laws of heredity have already yielded 

 a rich practical harvest in the improvement 

 of domestic animals and plants by selective 

 breeding. Naturally high hopes have also 

 been engendered as to the improvement of 

 the human species. But these hopes are 



hampered by enormous difficulties. Our 

 knowledge of human heredity is still rela- 

 tively meager in detail, and there is no agree- 

 ment as to what goals should be "bred for" 

 in any large-scale eugenics program. As to 

 an ideal feminine type, for example, college 

 men might insist upon the slim exotic beauty 

 of some current "pin-up" girl; whereas a 

 chauvinistic dictator might want to foster the 

 sturdy and fecund qualities of a peasant 

 mother. Moreover, mankind is averse to 

 tampering with individual rights and prefer- 

 ences; and it does not seem likely that 

 human matings will ever be governed, to any- 

 great extent, by the dictates of genetic prin- 

 ciple. Accordingly, the greatest hope for hu- 

 man advancement, at least in the near future, 

 still lies in the amelioration of the social, 

 educational, and economic conditions of hu- 

 man life. 



MUTATIONS 



All the facts of heredity emphasize the 

 stability of the genes, since the genes of a 

 species are generally transmitted without 

 change through many successive cell divi- 

 sions, and through many generations of the 

 organism as a whole. All the genes of the 

 zygote multiply successively as the many cells 

 of the developing embryo are formed, so that 

 every cell of the adult receives a duplicate of 

 every gene originally present in the zygote. 

 Moreover, every gene in the diploid germ 

 cells of the organism has an equal chance of 

 passing on into the next generation by way 

 of the gamete cells of the current generation. 



But the stability of genes is by no means 

 absolute. On rare occasion a certain gene may 

 change, suddenly and dramatically, into a 

 new and different gene. This important phe- 

 nomenon is called mutation.- As a result of 



2 Slrictly speaking, such a change of one gene into 

 another is referred to as a point mutation. Some 

 heritable changes may result from chromosomal phe- 

 nomena, such as translocation, duplication, or deletion 

 (p. 523), and these are called chromosomal mutations. 

 Presumably a point mutation involves a change in 

 the nucleotide sequence (p. 88) of the genie material, 

 which is the code of information transmitted by the 

 gene (p. 527). 



