遗传分析-第十七章Chromosome Mutation I Changes in Chromosome Structure.doc

Chapter 17 Chromosome Mutation I: Changes in Chromosome StructureKey Concepts Owing to the strong meiotic pairing affinity of homologous regions, diploids with one standard and one rearranged chromosome set produce pairing structures that have shapes and properties unique to that rearrangement. A deletion in one chromosome set is generally deleterious as a result of gene imbalance and the unmasking of deleterious alleles in the other chromosome set. Duplications can lead to gene imbalance but also provide extra material for evolutionary divergence. Heterozygous inversions show reduced fertility and reduced recombination in the region spanned by the inversion. A heterozygous translocation shows 50 percent sterility and linkage of genes on the chromosome involved in the translocation. Introduction Chromosome mutation is the process of change that results in rearranged chromosome parts, abnormal numbers of individual chromosomes, or abnormal numbers of chromosome sets. As with gene mutation, the term chromosome mutation is applied both to the process and to the product, so the novel genomic arrangements may be called chromosome mutations. Sometimes chromosome mutation can be detected by microscopic examination, sometimes by genetic analysis, and sometimes by both. In contrast, gene mutations are never detectable microscopically on the chromosome; a chromosome bearing a gene mutation looks the same under the microscope as one carrying the wild-type allele. Many chromosome mutations lead to abnormalities in cell and organismal function. There are two basic reasons for this outcome. First, the chromosome mutations can result in abnormal gene number or position. Second, if chromosome mutation includes chromosome breakage, which is often the case, then the break may occur in the middle of a gene, thereby disrupting its function. Chromosome mutations are important at several different levels of biology. First, in research, they provide ways of designing special arrangements