CHAPTER XIV 

 MUTATIONS 



Baur's third category of variations comprises all inheritable changes 

 due to causes other than segregation and recombination of genetic 

 factors. Although comparatively little is known concerning the specific 

 causes of mutations, yet it is possible to distinguish between two general 

 classes of such inheritable variations according to the nature of the 

 genetic units involved. These classes are (1) alterations in genetic 

 factors, and (2) deviations in the number of chromosomes. We designate 

 the first group as factor mutations and the second as chromosome aber- 

 rations. Since the first group is of vastly greater importance to agri- 

 culture than the second, we shall consider the latter very briefly before 

 engaging in discussion of the former, which we deem worthy of recognition 

 as mutations in the strict sense. 



Chromosome Aberrations. By the aid of cytology it has been demon- 

 strated that inheritable changes are occasionally induced, in plants at 

 least, by irregularities in the behavior of the chromosomes during mitosis 

 or meiosis, such that certain germ cells contain fewer or more chromo- 

 somes than the number typical of the species. Aberrant forms in 

 several plant families are now known to differ from the parent species 

 in chromosome number. Some have only a single chromosome more or 

 less than the parent, while a few are known in which the original number 

 is doubled. It is possible that aberrations occur involving all combina- 

 tions of numbers between these two extremes. In various forms of La- 

 marck's evening primrose ((Enothera lamarckiand) , whose typical number 

 is 14, according to Gates the following aberrant numbers have been 

 reported 15, 20, 21, 22, 23, 27, 28, 29, 30. Aberrations involving the 

 doubling of the number of chromosomes typical of the species is known 

 as tetraploidy because there are four times the haploid number typical 

 of the parent. Occasionally aberrations or hybridization between diploid 

 and tetraploid forms result in triploidy. 



There is a limited amount of evidence which indicates that groups of 

 species have arisen by progressive alterations in chromosome number. 

 Thus in Drosophila, Metz has found ten species in which the 

 chromosome numbers range from 6 to 12 and the larger numbers 

 appear to have arisen by subdivision of the large dumbbell-shaped 

 chromosomes found in the species having smaller numbers. Evidence 



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