136 GENETICS 



recessive character as do the parents. According to the 

 suggestion we arc considering, this is because the two X- 

 chromosomes in the offspring have the same parts modi- 

 fied, so that no corresponding dominant normal part is 

 present (figure 31, B). Thus, if the two parents have eosin 

 eyes, all the offspring have eosin eyes. 



There have been discovered certain cases in which two 

 X-chromosomes that are somewhat diversely defective do 

 not yield normal individuals when brought together. This 

 is true of certain recessive eye colors in Drosophila. If an 

 eosin-eyed mother is crossed with a buff-eyed father, the 

 daughters have both the modified X-chromosomes; but 

 their eyes are not of the normal red, but rather intermedi- 

 ate between eosin and buff. 



According to the suggestion we are considering, this 

 must be due to the fact that eosin and buff are due to re- 

 cessive modifications of the same part of the X-chromo- 

 somes, so that when the two are together, no corresponding 

 dominant normal part is present (figure 31, C), so that the 

 normal red color cannot be produced. Several other eye 

 colors have been found to act in this way in Drosophila. 

 There is a series of colors, ranging from white through va- 

 rious shades up to near the normal red, each color being 

 due to a modification of an X-chromosome. The different 

 colors have been named white, Ivory, buff, ecru, coral, eosin, 

 tinged, cherry, blood. When any two of these are brought 

 together in the same cell they give, not the normal red, 

 but various intermediate colors. According to the sugges- 

 tion we are considering, therefore, each of these must be 

 due to a modification of the same part of the chromosome. 

 Different X-chromosomes would have this particular part 

 modified In different ways, so as to yield different eye col- 

 ors. Is this a credible suggestion? 



As we shall see later, it is possible to get other evidence 



