Genetics 311 



one. An example of this type of inheritance is that of barring in do- 

 mestic chickens. The possible combinations of this character for each 

 sex are as follows : 



Male Female 



Z»Z« Barred Z«W Barred 



Z^Z" Barred Z'-W Nonbarred 



Z^Z" Nonbarred 



MUTATIONS 



In most of the genetic studies, the constancy of characters is em- 

 phasized and utilized. It is well known that a desirable character can 

 by careful selection be maintained through many generations. Occa- 

 sionally, however, this remarkable continuity is interrupted by the 

 sudden appearance of a new character in place of the old familiar one. 

 These sudden changes called "sports" have long been known to plant 

 and animal breeders, and were even utilized for the development of new 

 varieties. Such changes were not studied until after Mendel's work 

 was discovered, but in the years 1901 to 1903, a Dutch botanist, de Vries, 

 made intensive studies on the sudden changes that occurred in popula- 

 tions of evening primroses. He named these sudden changes ''muta- 

 tions" and suggested in the theory of mutations that new forms arose 

 by these sudden discontinuous changes. Until the gene theory as the 

 basis of heredity was formulated, the exact mechanism of these changes 

 remained mysterious. 



Gene Mutations. — The gene is at present considered to be a com- 

 plex protein molecule which has the unique ability to duplicate itself. 

 It is thought that slight changes in the arrangement of this molecule 

 may account for the observed phenotypic changes. 



These changes usually occur spontaneously and are inherited and 

 passed on to succeeding generations. In Drosophila, many such muta- 

 tions have been carefully studied. The many different eye colors have 

 arisen as mutations of the normal red. 



In the laboratory the rate of mutation among experimental ani- 

 mals has been greatly accelerated by the use of x-rays and other types 

 of radiation. It is presumed that these rays strike the individual genes 

 and cause rearrangements of the molecules. 



Mutations are recessive rather than dominant in most cases ; thus 

 they may not show up phenotypically for several generations. This 



