GENERAL ZOOLOGY 



Fig. 6.8. Left, Edmund Bcecher Wilson, 1856-1939. Right, Thomas Hunt Morgan, 1866-1945. 

 (Photographs courtesy Mrs. A. F. Hucttner.) 



fruit fly, Drosophila. An example of sex-linked inheritance may be selected 

 from the abundant data concerning heredity in this small insect. Red eye 

 color is dominant to white eye color in Drosophila (Fig. 6.9). When a red- 

 eyed female is crossed with a white-eyed male, both male and female offspring 

 of the F\ generation are red-eyed. If such red-eyed individuals are inter- 

 crossed, all the females and one-half of the males of the F,, generation have 

 red eyes, whereas one-half of the males have white eyes. The reciprocal cross, 

 or the cross between a white-eyed female and a red-eyed male, gives very 

 different results (Fig. 6.10). The males of the F^ generation are white-eyed, 

 and the females are red-eyed. In the F2 generation red-eyed and white-eyed 

 males and females occur in equal numbers. When the mechanism of inherit- 

 ance is considered in the following section, sex-linked inheritance will be 

 found to furnish additional confirmation of Mendelian principles. 



The method of experimental breeding, first carefully used by Mendel, has 

 yielded a vast amount of detailed information concerning the course of in- 

 heritance of specific characteristics by particular individuals through succes- 

 sive generations. It was clear to Mendel that the reproductive cells, which do 

 not, of course, exhibit the characters of stems and seeds, must carry something 

 correlated with the appearance of characters of the adult organism from 

 generation to generation. These "somethings" are usually called hereditary 



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