288 Heredity and Eugenics 



stand for the single male sex chromosome, which, by hypothe- 

 sis, lacks the color distinguishing factor. Let the white 

 disk (W) symbolize the absence of a chromosome in a germ 

 cell. Let the black disks (B) represent the two female 

 sex chromosomes with the factor of color-sight. Then the 

 union of S-\-B gives the female sex and has the determiner 

 for color, albeit simplex. The union W-\-B gives the male 

 sex and also has the determiner for color-sight. Hence 

 .* neither sons nor daughters of a color- 



* blind man are color-blind. If the son 



5 K^i 4^ , . 



V B marry a normal woman, it is clear 



w Q ^ that (since no S comes into the union) 



the children are normal. But if the 



FIG. 98.-Diagram illus- daughters marry half Q f the males 



tratmg method of inherit- ' ' 



ing sex-limited characters, will receive the single S chromosome 



The circles represent sex anc j g^ will fog color-blind. Thus 



chromosomes. ,11 f i i A > P 



the long famous knight s move form 



of heredity of color-blindness is explained. Several other 

 traits are inherited in the same way: bleeding, imperfect 

 development of the iris, and atrophy of the optic nerve. 

 In all these cases unaffected males may marry with impun- 

 ity; but females of the strain who have affected brothers 

 should not have children. 



The foregoing considerations bring clearly to mind the 

 great advance that has been made hi recent years in the 

 analysis of the inheritance of traits. At last it is possible 

 to give definite advice to those about to marry, or who do 

 not wish to transmit their undesirable traits. Of the 

 method of inheritance of many traits we are still in ignor- 

 ance. In the absence of detailed knowledge, the best gen- 

 eral advice that can be given is this; marry dissimilars. 

 Weakness in any trait should marry strength in that trait; 

 and strength may marry weakness. 



