310 ANIMAL BIOLOGY 



afford the material for recombinations. Not infrequently relatively 

 radical alterations, or mutations, occur, usually just before or 

 during the maturation of the germ cells, which thereupon are at 

 the disposal of the usual genetic mechanism for segregation, etc. 



Mutations may be broadly classified as chromosomal aberra- 

 tions and intrinsic changes in the individual gene. Indeed some 

 geneticists restrict the term mutation to gene changes. A few 

 illustrations from the wealth of available data must suffice. 



Chromosomal aberrations consist of departures from the 

 normal number and arrangement of the chromosomes, and of 

 their parts. Thus many cases are known in which the whole nor- 

 mal chromosome set (diploid number) has been increased to some 

 multiple of the typical haploid number. Such symmetrical changes 

 in the chromosome groups are termed ploidy, and appear to be 

 more frequent in plants than in animals. Thus the haploid sets of 

 three well-known varieties of Wheat consist of 7, 14, and 21 chro- 

 mosomes respectively. 



Some marked instances of the origin of new types have recently 

 been observed in the Jimson weed (Datura). In one case the plants 

 differ in size and several other characters that clearly are attribut- 

 able to the doubling of the diploid number of chromosomes. They 

 have 24 pairs of chromosomes instead of the typical 12 pairs. 

 Equally interesting are certain observations on Drosophila which 

 typically has 4 pairs of chromosomes. Individuals have appeared 

 that differ markedly from the normal flies and possess 12 chromo- 

 somes as a result of the addition of another haploid set. 



Another type of chromosome mutation, known as hetero- 

 ploidy, involves only one, or rarely two, chromosomes, and not an 

 entire set. Thus again in Drosophila, the failure of the X chro- 

 mosomes to separate after synapsis (non-disjunction) gives rise 

 to individuals with nine chromosomes, including one Y and two X 

 chromosomes. This may be regarded as a 'male' complement of 

 chromosomes plus another X chromosome, with the result that 

 flies so endowed are females with an altered heredity visible chiefly 

 as larger size. (Fig. 197.) 



Still other irregularities may involve only a portion of one chro- 

 mosome. Thus part of a chromosome may be lost, or duplicated, 

 or attached to another chromosome. For example, a race of 

 Drosophila has been obtained in which a part of the Y chromo- 

 some has become transferred (translocated) to the end of an X 



