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Modern Genetic Analysis Chapter 7. Gene Mutations
Introduction

Genetic analysis would not be possible without variantsorganisms that show phenotypic differences in some particular character. In previous chapters we performed many analyses of the inheritance of such variants; now we consider their origin. How do genetic variants arise?

The simple answer to this question is that organisms have an inherent tendency to undergo change from one hereditary state to another. Such hereditary change is called mutation. As we shall see, the term mutation covers a broad array of different kinds of changes. In the next chapter, we shall consider mutational changes affecting entire chromosomes or large pieces of chromosomes. In the present chapter, we focus on mutational events that take place within individual genes. We call such events gene mutations.

This chapter covers two main topics. First, we consider the kinds of gene mutations that can occur at the DNA level and the consequences of these DNA changes on protein structure and function. Important components of this discussion are the molecular mechanisms whereby such mutations arise. Of particular interest is the ability of certain agents to increase the rate at which mutations occur. These agents are useful experimentally, but interest in them also stems from fears that humans may be exposed to such agents in everyday life. We also need to consider possible molecular mechanisms for the much less frequent, but evolutionarily more important, mutations that occur "spontaneously." In these discussions, we shall see that all cells have evolved sophisticated DNA repair systems that prevent mutations from occurring. Thus, DNA can be viewed as undergoing a dynamic tug-of-war between the processes producing damage that can lead to new mutations and the cellular repair processes that are constantly searching for and eliminating premutational events. The second main topic of the chapter is the methodology of mutational analysis. Every mutation obtained by the experimenter represents a component of the biological process under study. We shall see how a specific mutation that is of interest is picked out from a sea of other mutational changes. Crucial in this process is the ability to know which aberrant phenotypes will reveal the mutations that are most relevant to the task at hand.

© 1999 by W. H. Freeman and Company. All rights reserved.