Chromosomes, which contain genetic information in the cells of many different organisms, can become mutated through a variety of different processes, often to the detriment of the mutated organism. A chromosome mutation takes place at the chromosome level, meaning that the whole structural unit of the chromosome is altered in some way. A chromosome mutation is often considered to be different from a gene mutation, in which only a single gene on a chromosome is changed by a mutation. Chromosome mutations occur on a larger scale that affects a significant part of the whole chromosome, so many genes may be affected by a single mutation.
A chromosome mutation is generally classified based on the particular structural change made to the chromosome or chromosomes. One type of mutation, for instance, is a fusion; it occurs when two different chromosomes or chromosome segments fuse together into one. Researchers actually believe that the human second chromosome is a fusion of two chromosomes possessed by pre-human ancestors. Another type of chromosome mutation is referred to as an inversion and occurs when a segment of a chromosome is inverted. Inversions often do not cause any visible mutations to the organism, as all of the genetic information is generally intact and unaltered — though this is not always the case.
When a chromosome mutation alters the number of copies of a particular gene in an organism's genome, it is much more likely to cause some noticeable or significant effect on the organism. Common mutations of this form include insertions, which insert a new segment into a chromosome, and deletions, which remove a segment from the chromosome. Both of these types of chromosome mutations result in a change in the copies of a gene or genes present. Overexpression or underexpression of a gene, either of which can occur from such mutations, can both cause drastic effects on gene expression within an organism. In the white mutation in the fruit fly Drosophila melanogaster, for instance, failure to sufficiently express a gene coding for eye pigmentation results in flies with white eyes.
Researchers have many different ways of identifying a particular chromosome mutation or gene mutation. Organisms with different genetic traits can be crossed in different combinations in order to develop a general "map" suggesting the location of a given mutation. Researchers can also sequence parts of the organism's genome. Sequencing presents researchers with a detailed view of an organism's genetic information and its location on a chromosome.