遗传分析-第十五章.docVIP

Chapter 15 Gene MutationKey Concepts Mutation is the process whereby genes change from one allelic form to another. Forward mutations are changes away from the wild-type allele, and reverse mutations are changes to the wild-type allele. Mutations can lead to loss of function of a gene or to new function. Mutations in germ-line cells can be transmitted to progeny, but somatic mutations cannot. Selective systems make it easier to obtain mutations. Mutagens are agents that increase normally low rates of mutation. Genes mutate randomly, at any time and in any cell of an organism. A biological process can be dissected genetically if mutations that affect that process can be obtained. Each gene identified by a mutation identifies a separate component of the process. Introduction Genetic analysis would not be possible without variants—organisms that differ in some particular character. We have considered many examples in which we could analyze differing phenotypes for particular characters. Now we consider the origin of the variants. How, in fact, 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. Geneticists recognize two different levels at which mutation takes place. In gene mutation, an allele of a gene changes, becoming a different allele. Because such a change takes place within a single gene and maps to one chromosomal locus (“point”), a gene mutation is sometimes called a point mutation. At the other level of hereditary change—chromosome mutation—segments of chromosomes, whole chromosomes, or even entire sets of chromosomes change. Gene mutation is not necessarily a part of such a process; the effects of chromosome mutation are due more to the new arrangements of chromosomes and of the genes that they contain. Nevertheless, some chromosome mutations, in particular those proceeding from chromosome breaks, are acc