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 (Znothera lamarckiana), 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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