24 



GENETICS 



the father has the bar-eye. In figure 9 are shown the head 

 and eyes of the two parents. The figure shows also the 

 X-chromosomes of the two parents. The mother's two X's 

 are shown in black, while the father's single X is shown 



Figure 8. The fruit-fly or banana-fly, Drosophila melanogaster, on 

 the study of which much of our knowledge of heredity is based. Fe- 

 male at the left, male at the right. After Morgan, Bridges and 

 Sturtevant, The Genetics of Drosophila. 



in white; this makes it possible to follow the father's X in 

 the later generations. 



We know that in the next generation the sons have no X 

 from the father, while all the daughters contain one from 

 the father (as well as one from the mother). And it is 

 found that the daughters have the bar-eyes, like those of 

 the father, while the sons have not (figure 9). 



Thus all the children that get an X-chromosome from 

 the father have eyes like the father, while the rest do not. As 

 there may be 200 offspring in a family of the fruit-fly, this re- 

 sult is very striking. A hundred daughters, having their fa- 

 ther's X, have also their father's eyes ; a hundred sons, lacking 

 their father's X, lack also his type of eyes. 



