52 GENETIC VARIATIONS 



diflferent seeds, and these colors are (partly) heritable in later 

 generations. 



Thus in these plants the genes frequently mutate so as to 

 yield at one time one color, at others another. The mutations 

 occur in many different ways, yielding many different colors 

 and patterns of variegation. 



These facts as to frequent, reversible and inconstant muta- 

 tions raise questions as to the essential nature of mutations. 

 What is it that is really occurring here ? To this question we. 

 return. 



What are the agents that produce mutation? For many 

 years after mutations w^ere found to occur, nothing was 

 known as to the agents that cause them. A few genes mutated, 

 as a few of the atoms of a radiant metal disintegrate; in both 

 cases no outside agent appeared to be at work. There seemed 

 to be no relation between the environment and the produc- 

 tion of genetic variations. 



But of late this situation has changed. The connection be- 

 tween environmental action and genetic variation has been 

 made. Certain outer agents are found to play a role in produc- 

 ing gene mutations. In 1927 H. J. MuUer' discovered that by 

 subjecting developing organisms to a powerful dose of X-rays, 

 the genetic system was altered in many ways. The X-rays 

 injure the animals; they kill a large proportion of them; they 

 leave many of the rest sterile. Still other survivors are found 

 to have suffered changes that leave their descendants with 

 hereditary abnormalities, weaknesses and other peculiarities; 

 and these are found to be the result of changes in the genetic 

 system caused by the radiations, changes that are known as 

 gene mutations. The radiations have caused the chromosomes 

 to break, caused pieces from different chromosomes to stick 

 together; destroyed some of the genes; produced irregularities 

 of many kinds. And amid all this wreckage, they have pro- 



