Chapter 15 

 THE NATURE OF GENE MUTATIONS 



It is difficult to state exactly how a gene mutation occurs until 

 more is known of the structure and chemistry of the genes them- 

 selves. If genes can be pictured as molecules or as large chemical 

 radicals that are attached to an undifferentiated thread, they are 

 probably complex radicals with numerous side chains. One the- 

 ory suggests that the mutation is the result of a rearrangement 

 of some of the atoms of the molecule or radical which constitutes 

 the gene. It is also possible, as has been suggested, that a part 

 of a molecule might be lost. According to these views, the gene 

 is a more or less distinct entity with its own complex structure 

 and a mutation is merely a loss or rearrangement of intramolecu- 

 lar material. According to another theory the chromosome as 

 a whole is to be regarded as the unit of heredity and the so- 

 called gene mutations are merely rearrangements of segments 

 of the chromosome. This rearrangement is discussed further in 

 Chapter 17. 



The kinds of changes that mutations may produce in the 

 phenotype of an organism are countless. In both plants and 

 animals, all parts of the organism may be affected, and each 

 part may be affected in many different ways. Mutations are 

 known which affect the shape, size, and color of almost every 

 known organ of plants and animals, and some have been found 

 which affect even such characteristics as the stability of another 

 gene, the mutation rate of other genes, spindle formation at 

 meiosis, viability of gametes, incompatibility of certain eggs 

 and sperm, and other similar fundamental biological processes. 



Finally, in many organisms, mutations have arisen which lead 

 to the death of the individual by upsetting normal embryological 

 development or by affecting certain fundamental organs or phy- 

 siological processes. 



Although some mutations produce rather large and striking 

 effects, many mutations produce small effects, as was shown by 



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