310 INTRODUCTION TO CYTOLOGY 



It is usually assumed as a workable hypothesis that a gene may exist 

 in two or more allelomorphic states. ^^ It is only when at least two 

 states are available that the function of the gene can be conveniently 

 studied in crosses. Hence the genes used in genetic studies and appearing 

 in chromosome maps are "mutant genes," i.e., genes which in some cases 

 have become "mutated," or altered from a former condition, so that the 

 effects of two allelomorphic states can be compared. A large share of 

 the responsibility for the production of a given character may lie with 

 uniform and unknown genes. 



The term mutation may be used in a general sense for all types of 

 alteration in the genotype aside from that accomplished by normal 

 meiosis and syngamy, the types being classified on the basis of the 

 amount of the substance involved (gene; group of genes or section of 

 chromosome; chromosome; chromosome set) and the nature of the 

 process causing the alteration (Bridges, 1923). There is a tendency 

 among some geneticists, however, to restrict the term to gene mutations. 

 The hypothesis that a comparatively stable gene may under certain 

 occasional circumstances (perhaps as it reproduces) undergo a qualitative 

 change with an effect upon one or more characters underlies much of the 

 genetic investigation now in progress. It is supposed that such an altera- 

 tion may involve a single locus or gene ("point mutation") or a group of 

 linked genes occupying a certain region of a chromosome ("regional 

 mutation," or "complex mutation"); but it is not clear in many cases 

 whether the change is strictly a qualitative alteration in the gene, and 

 therefore a true gene mutation, or a mere derangement of a very small 

 region of the chromosome. 



There is genetic evidence to show that gene mutations may occur at 

 any place in the organism or stage of the life cycle, but they are most 

 frequent in the germ cells at or near the meiotic period. ^^ Ordinarily, 

 therefore, only a few of the many reproductive cells transmit the altered 

 gene. When, as seems to be true of the majority of known cases, the 

 mutation is from a dominant to a recessive condition, the effect on the 

 characters of the diploid organism is not fully manifested until two 

 such recessive allelomorphic genes are brought together. Mutations 

 occurring in body cells are known as "somatic mutations." Two of the 

 most conspicuous results of somatic mutation in plants are bud sports 

 and chimeras. If the mutation occurs in the meristematic tissue of a 

 very young bud, the product of the bud — branch, flower, or flower cluster 

 18 When more than two are known, they are called "multiple allelomorphs." In 

 Phytodeda, for example, the color of the beetle is affected b}^ such a series of four 

 allelomorphs: N, R, A, L. Each of these is dominant to any one on its right in the 

 order stated (Zulueta, 1925). 



i» Emerson (1914, 1917, 1922), Eyster (1924), Bridges (1919), MuUer (1920), 

 Blakeslee (1921), Anderson and Eyster (1928), MacArthur (1928), Gowen (1929), 

 Demeree (1929d). 



