20 GENETICS 



In still other species, the special chromosome that the 

 male has — the Y-chromosome — Is not smaller than the X 

 of the female, but Is of a different shape. This Is the case 

 for example In the fruit-fly (Drosophlla melanogaster), 

 which has been studied In respect to these matters more 

 thoroughly than any other organism. In this animal the 

 Y-chromosome Is hook-shaped, while the corresponding 

 X-chromosome of the female is straight (see figure 7). In 



Female Male 



Figure 7. The two groups of chromosomes in the fruit-fly, Droso- 

 phila melanogaster. The female group at the left, with two straight 

 X-chromosomes. The male group at the right, with one straight X 

 and one bent Y. After Morgan, Bridges and Sturtevant, The Genet- 

 ics of Drosophlla. The Roman numerals I to IV are the designations 

 commonly employed for the different pairs, the two X's, (or X and 

 Y) constituting pair I. 



such cases physiological study shows (as will later appear) 

 that the Y-chromosome is almost without function. 



There Is thus a series of gradations among different 

 species. The males have one chromosome that, in different 

 cases, is Inactive, small, or entirely lacking, taking the place 

 of a fully developed functional chromosome (X) that Is 

 present in the female. The loss of the activity of this 

 chromosome In some way causes the egg to develop Into 

 a male instead of a female. What the nature of this physi- 



