162 GENETICS AND EUGENICS 



respectively, as actually observed in about one case in a hun- 

 dred by Morgan, 



It may add interest to the case to state parenthetically that 

 in man occur a number of sex-linked variations which are in- 

 herited in this same curious fashion. Among them may be 

 mentioned color blindness and bleeding {haemophilia), which 



TABLE 22 

 Reciprocal Crosses of White-Eyed and Yellow-Bodied Flies 



occur chiefly in males, but are never transmitted by males to 

 their sons but only through their daughters to their grand- 

 sons. 



Morgan and his pupils have described between forty and 

 fifty characters in Drosophila which are sex-linked in hered- 

 ity; they also have discovered a large number of other 

 Mendelizing characters in Drosophila which are not sex- 

 linked but which nevertheless are inherited in groups, char- 



%(<^ 



\« 



Fig. 118. Drawing showing the four pairs of chromosomes seen in the 

 dividing egg-cell of Drosophila. (After Dr. C. W. Metz.) 



acters in the same group showing coupling when introduced 

 in a cross from the same parent, and repulsion when intro- 

 duced from different parents. The number of these groups 

 exactly corresponds with the number of the chromosomes and 

 Morgan believes that their genes are located in the chromo- 

 somes, an hypothesis which seems reasonable but which 

 would be severely strained if an additional group of characters '^ 

 should be discovered. There are three groups of the non-sex- 

 linked characters. (See Fig. 119.) In one of these referred 

 to as Group II (the sex-linked group being called Group I), 



