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 of genes, uniquely suited to answer certain biological questions. Second, chromosome mutations are important at the applied level, especially in medicine and in plant and animal breeding. Finally, chromosome mutations have been instrumental in shaping genomes as part of the evolutionary process.

Let’s consider some important properties of chromosomes that are useful in understanding structural chromosome mutations.

1.

In prophase I of meiosis, homologous regions of chromosomes have a strong pairing affinity and, if necessary, will contort to pair. Consequently, many curious structures may be seen in a cell that has one standard set of chromosomes and one aberrant set. Remember that in polytene chromosomes the homologs also pair (even though they are not in meiotic cells), so comparable shapes result.

2.

Changes in structure are usually due to chromosome breakage, and the broken chromosome ends are highly “reactive,” tending strongly to join with other broken ends. The telomeres (the regular chromosome ends), however, do not tend to join.

3.

In a diploid, if parts of chromosomes are lost or gained, the result is almost always lethal. The chromosome set is exquisitely sensitive to changes in gene content, even if one complete set is present. Examples of such disturbed gene balance will appear throughout this chapter and the next.