MATERIALS OF HEREDITY 23 



5. The offspring that get the father's X-chromosome 

 become females (since they have also an X-chromosome 

 from the mother). 



4. The offspring that get the father's Y-chromosome 

 become males. They get their single X-chromosome from 

 their mother. 



5. Hence, the father's X-chromosome always passes to 

 his daughters, never to his sons. 



6. The sons get their one X-chromosome exclusively 

 from the mother. 



7. Daughters get one X-chromosome from the mother, 

 one from the father. 



We know therefore that in the next generation the 

 father's X-chromosome will be found in his daughters, not 

 in his sons. So if the father's X-chromosome produces some 

 distinctive effect, this effect will be seen in his daughters, 

 not in his sons. And since the rules given above hold also 

 in later generations, the father's X (as distinguished from 

 the mother's) may be traced for generation after genera- 

 tion, and its distinctive effects observed. In the children or 

 grandchildren an X from the father can be substituted in 

 place of one from the mother. 



An example will show how this works; it will be taken 

 from the fruit-fly, Drosophila (figure 8), in which these 

 matters have been studied most thoroughly.^ This is the 

 small insect often seen flying about bananas or other fruits. 

 It is in many respects the classic organism for genetics. 



In the fruit-fly the compound eye usually has a great 

 number of units or facets, gathered into an oval shape 

 (figure 9). But sometimes there are individuals in which 

 the eye is abnormal. In such cases many of its outer facets 

 are incompletely developed, so that there is merely a broad 

 bar of complete facets across the eye (see the figure) ; this 

 is spoken of as "bar-eye." For the experiment a mother is 

 selected that has the full number of normal facets, while 



