208 THE SCIENCE OF BIOLOGY 



shading a portion of one of the "X" chromosomes (Fig. 27). 

 Color blind men are known to be much more numerous than 

 color blind women. But color blindness is transmitted by a 

 human male through his daughters to approximately one-half 

 of his grandsons. 



Explanation of this very peculiar inheritance is as follows : 

 A color blind man is mated with a normal woman (Pi, 

 Fig. 27) . The single sex chromosome of the man carries the 

 factor for color blindness, as shown by the shading. One- 

 half of the Pi germ-cells of the male will be free of the defect, 

 since they possess no "X" chromosome. In the Fi genera- 

 tion, the males are all normal and free from the defect. But 

 all the females have an "X" chromosome (partly shaded) 

 which carries the defect. For some reason such a " single 

 dose" will not produce color blindness in a woman although 

 it does so in a man. These F x females transmit the chromo- 

 some bearing the defect in the manner shown (F 2 ), so that, 

 where numbers are sufficient to indicate the ratio, one-half 

 the grandsons of such matings are color blind and the other 

 half normal; while the granddaughters, although all seem 

 normal, are one-half free of the defect and one-half trans- 

 mitters of color blindness as the figure shows. Color blind 

 women result from matings in which two "X" chromosomes, 

 each with the color blind factor, are brought together. Such 

 a combination would be possible in one-half the daughters 

 arising from color blind men mated with women who were 

 transmitters, i. e., possessed a single dose of the defect. 20 



These results which have been obtained from the study of 

 sex linked characters, find their explanation in the work of 

 cytology. The latter science has been mainly observational. 

 The cytologist is now stimulated to extend his analysis by 

 means of experimentation. Hybridization offers an oppor- 

 tunity to produce new chromosomal combinations by ex- 

 perimental crossing. The observational foundation is suf- 

 ficient for the beginning of such experimentation. 



20 Morgan, T. H., "Heredity and Sex." CJ. for extended discussion. 



