Chap. 20 THE PHYSICAL BASIS OF HEREDITY 397 



Fig. 20.7. The fruit fly, Drosophila melanogaster. This and other species of these 

 common gnat-like flies have contributed more material to the study of genetics than 

 any other animal. They have 8 chromosomes in the body cells but the genes con- 

 tained in them are responsible for thousands of structures and actions. Fruit flies 

 were used by Thomas Hunt Morgan in his studies which constitute some of the 

 most important contributions to the science of genetics. (Courtesy, Morgan: The 

 Physical Basis of Heredity. Philadelphia, J. B. Lippincott Co., 1919.) 



Genes act like enzymes in that they are able to speed up or slow down 

 chemical actions without themselves being used up in the process. They re- 

 semble viruses in being extraordinarily minute. Like them they multiply only 

 within living cells; they have specific effects upon cells; they may change 

 (mutate) in nature; and may be caused to change by exposure to x-rays. 

 Genes and viruses differ, genes being orderly and mainly beneficial, while 

 viruses, at least the well-known ones, are usually lawless and destructive. 



Genes are inherited but some of the characters which they control may be 

 modified by environment. In man, the ability to smile is inherited, but not the 

 exact smile for those of fat faces differ from those of thin faces and food may 

 create the change. 



the content of the sperm cell that happens to join the egg from which he or she 

 develops, xx a female, xy a male. Grasshopper, cells of the body (except sex cells); 

 male has 21 and female 22 chromosomes. Every egg has an x chromosome. Half 

 of the sperm cells have an x chromosome, and half of them have no .v chromosome. 

 If the latter fertilizes an egg it produces a male (20+ a:) grasshopper. Domestic 

 fowl, cells of the body (except sex cells) : male and female each have 18 chromo- 

 somes. In males the pair of sex cells are called zz; in females the members of this 

 pair are different and called zw. 



Every sperm contains the z chromosome; half of the eggs contain z and the other 

 half contain the w chromosome. A z sperm fertilizes an egg with the z chromo- 

 some and produces a female zz. Honeybees, fertilized eggs (sperm 16 chromo- 

 somes and egg 16), having the diploid number of 32 chromosomes develop into 

 females (queen and workers); unfertilized eggs (no sperm, and egg 16 chromo- 

 somes), having the haploid number of 16 chromosomes develop into males 

 (drones). Their body cells have only half the number of chromosomes (16) that 

 are contained in the body cells of the females (32). (Courtesy, Winchester: 

 Genetics. Boston, Houghton Mifflin Co., 1951.) 



