GENETICS IN THE SERVICE OF MAN GLASS 307 



lized egg from which each of us starts out in life must develop those 

 characteristics common to all members of its species, race, and family, 

 besides those peculiar to itself. One inherits only what is in the ferti- 

 lized Qgg : that is, in the physical sense, only the chromosomes and the 

 genes they contain ; in the developmental sense, only the potentialities 

 and capacities inherent in those genes. Those potentialities can be 

 realized only within the limitations of the environment. 



Heredity is biparental ; for each gene inherited from one's mother, 

 a corresponding gene is inherited from one's father. These corre- 

 sponding genes are not always identical, because mutations give rise 

 to varieties of the same gene within the population. Should the two 

 genes of a pair happen to be different (a condition called "heterozy- 

 gous"), then one gene, known as the dominant, commonly determines 

 the specific trait concerned; and the other gene, the recessive, is 

 masked, although it will still be transmitted to succeeding generations 

 without alteration. Thus a person who has a pair of genes, one for 

 brown hair and the other for red, will have brown hair, and the 

 presence of the gene for red hair may be quite unsuspected. One can 

 have red hair only by inheriting two genes for red hair, one from each 

 parent; one would then be "homozygous." A good many of each 

 person's genes are probably recessive, and most of these will be 

 heterozygous and as a consequence unobservable. 



The significance of this for eugenics is unmistakable : a considerable 

 proportion of the genes in any person, and therefore in the entire popu- 

 lation, is hidden, and to that extent the measures of eugenics must be 

 applied blindly. 



Many characteristics are determined or affected by more than one 

 pair of genes — intelligence, for example. In such a case it is a partic- 

 ular combination of dominant genes, perhaps together with some 

 homozygous recessive genes, that determines the nature of the char- 

 acter. These combinations are very rarely inherited in toto^ the reason 

 being that no one inherits all his parents' genes, but only half of the 

 genes of each. Pure chance determines which gene of the two making 

 up each pair in the father and the mother will be transmitted to the 

 child. Hence, the more genes there are in a given combination, the 

 lower the chance that that combination will be transmitted intact. 

 Nearly all combinations are broken up as the germ cells mature, and 

 new combinations of a biparental origin are formed. Inasmuch as 

 most of the human characteristics which are of social significance de- 

 pend upon multiple genes, the combinations of which cannot be pre- 

 served, whether good or bad, the importance of this reassortment and 

 recombination of genes in heredity to the success of eugenics can 

 scarcely be overemphasized. Moreover, a particular gene which in 

 most combinations has an adverse effect may, in just the right combina- 



