GENETIC VARIATIONS 34 1 



herltance. It must be shown further that it is not a mere 

 result of change of grouping of the genes or chromosomes, 

 such as are described in the preceding chapter. It must be 

 shown that the change in characteristics is due to change 

 in action of a single gene. This is done by a study of the 

 method of inheritance, by proof that the inheritance is of 

 the "single factor" or single-gene type, described in pre- 

 vious chapters. These conditions are not easy to fulfill, and 

 recent discoveries, described in the preceding chapter, have 

 much shaken the certainty of the adequacy of such proof. 

 According to one view — the view that some of the so-called 

 mutations are "position effects," mentioned on page 335, 

 — changes that satisfy all tests for gene mutations may re- 

 sult from mere alteration of the relative position of certain 

 genes. On the other hand, the commoner view is that the 

 mutations are essentially alterations in the chemical or phy- 

 siological constitution of the single genes. 



Reserving for later pages the question as to the essential 

 nature of mutations, we examine their occurrence and the 

 genetic phenomena which they present. Mutations occur in 

 organisms subjected to certain conditions, and also in or- 

 ganisms left under natural conditions. We deal first with 

 those occurring under natural conditions. 



To ordinary day-by-day observations, mutations appear 

 to occur very infrequently.^ Yet when large numbers of in- 

 dividuals are examined for long periods, and the eye is care- 

 fully trained for the detection of mutations, the number 

 occurring in even a short period of years is considerable. In 

 the fruit-fly, during twenty-five years of observation, mu- 

 tations have been seen to occur in several hundred different 

 genes. The gene mutations have occurred in genes that affect 

 all parts and functions of the organism, so that, as before 

 observed, practically all knowledge of heredity in this or- 

 ganism is based on the mutations that have occurred. The 

 ordinary "wild" individuals are extremely uniform. Mating 



