Chap. 20 THE PHYSICAL BASIS OF HEREDITY 405 



fertilized by such O-sperms and appear typically male. However, breeding 

 experiments have proven that these males are sterile. In other species in 

 which half the sperms regularly lack a sex chromosome (e.g., grasshoppers), 

 the males are fertile. Y-chromosomes contain only a few genes. In fruit flies 

 these appear to be associated with fertility. In the XO male fruit flies that is 

 the main character missing. 



Different as males and females are, they are also fundamentally similar. 

 Some invertebrates require but a slight shift in conditions, perhaps of the 

 genes, to tilt the organism toward maleness or femaleness. Sometimes ab- 

 normal chromosome numbers resulting in a different balance of the genes 

 may produce supermales, superfemales, or intersexes as in Drosophila (Fig. 

 20.14). Higher animals are seldom if ever entirely male or entirely female, 

 as the nipples of human males bear witness. The possible explanation may 

 be that every individual carries all the genes essential for both sexes and 

 that certain genes or conditions of the genes tip the balance toward maleness 

 or femaleness. 



Discovery of Sex Chromosomes. Sex chromosomes were first correctly 

 interpreted fifty years ago (1901) by C. E. McClung during his study of the 



Fig. 20.14. Sex types in fruit flies, Drosophila. Upper left, normal female; upper 

 right, intersex; lower left, supermale; lower right, superfemale, and chromosomes 

 of each type. (After Bridges. Courtesy, Snyder: Principles of Heredity, ed. 4. 

 Boston, D. C. Heath and Co., 1951.) 



