MENDELISM AND MUTATION 351 



one or two original types, and that the specific differences exhibited by 

 the organisms are related to these differences in their chromosome 

 complements. 



This conclusion is supported by the observations of McClung, Robert- 

 son, and others on the chromosomes of other insect families. In the 

 grasshoppers, for example, Robertson (1916) finds that the various 

 chromosomes of the complement form a regular graded series and 

 can be identified in all the species and genera studied. The nearer the 

 relationship the more nearly similar are the chromosome complements. 

 Robertson states that the degree of relationship is as clearly expressed 

 in the nucleus as in the externally visible characters, and that the 

 evidence indicates that descent by variation from a common ancestral 

 series of chromosomes is paralleled by degrees of variation in somatic 

 structures. 



Mutations Accompanied by No Change in Chromosome Number. 

 In most of the examples of this class it has been found that the mutant 

 behaves in a strictly Mendelian manner, usually being recessive to the 

 type from which it sprang. This observation falls into line with the fact 

 that the number and behavior of the chromosomes remain the same; the 

 operation of the Mendelian mechanism is not disturbed. Consequently 

 if the origin of mutations of this class is dependent on the chromosomes 

 it must be due to a change of some kind occurring within the chromosome 

 and affecting the character of its factors, or genes. That such factor 

 mutations do take place is the hypothesis upon which a large school of 

 geneticists is attempting to account for many of the observed phe- 

 nomena of inheritance. Such mutations may involve either a single 

 gene only ("point mutation") or a group of genes occupying a given 

 region of a chromosome ("regional mutation"). Furthermore, they 

 may apparently occur either in the germ cells or in the somatic cells, 

 but seem to be most frequent in the former at the time of maturation, 

 for the reason that only one or two gametes (or spores followed later by 

 gametes in most plants) among the large number produced reveal the 

 presence of the altered gene in their effect upon the offspring. The 

 mutation in the gene must here take place after the multiplication of 

 the germ cells has been nearly or quite completed; otherwise the effect 

 would be manifested by a larger number of gametes (or spores). If, as 

 is true of the majority of cases, the mutation is such as to result in a 

 recessive character, this character does not manifest itself until it meets 

 a similarly mutated gene in the homozygous individual. Thus such 

 an alteration may remain latent for many generations, or may never 

 come to expression at, all. 



Factor mutations occurring in somatic (meristematic) cells result in 

 what are known as " vegetative mutations." These are of two principal 

 kinds: bud sports and chimeras. In the case of the bud sport, which is 



