THE METHOD OF EUGENICS 23 



male X chromosome. Its inheritance can be followed from 

 Figure 7, adapted from Wilson, 1911. 



If the trait be a positive sex-limited one, originating 

 either on the father's or the mother's side, its inheritance 



gametes X 



zygotes 

 gametes 



zygotes XS 



1 2 



Fig. 7. — Diagram illustrating the method of inheritance in sex limited 

 heredity. A', the sex chromosome, double in the female individual, single in 

 the male. When ripe germ cells are formed in the female, each contains the sex 

 determiner, but in the male half of the germ cells have and half lack the deter- 

 miner (represented by the dash — ). Let X' represent the sex chromosome of 

 the original male that showed the defect (absence of some unit character). 

 Let such a male be mated with a female of an unaffected strain. Then all 

 children will have the determiner for the positive condition (Gen. 2, zygotes, 

 i. e., fertihzed eggs and the individuals that develop from them). In the third 

 generation four kinds of zygotes will appear: 1, the normal female who is not 

 capable of transmitting the defect; 2, the normal female who is capable of 

 transmitting the defect; 3, the normal male who is incapable of transmitting 

 the defect; 4, the defective male. Baaed on E. B. Wilson, IQIL 



will be more irregular; but it can be worked out by the aid 

 of Figure 7. 



6. The Application of the Laws of Heredity to 



Eugenics 



If one is provided with a knowledge of the methods of 

 inheritance of unit characters it might seem to be an easy 

 matter to state how each human trait is inherited and to 

 show how any undesirable condition might be eliminated 

 from the offspring and any wished for character introduced. 



