Deletions are rearrangements which involve the loss of regions of a chromosome. Deletions are normally lethal if large and in homozygous condition. They can be detected at the genetic level through crosses to recessive mutations. Deletions allow recessive alleles to be "unmasked" and expressed if they are located within the deleted region. Deletions can be detected cytologically in metaphase chromosomes if a large region is lost.
The chromosomes in Drosophila salivary glands are "polytene", i.e., have duplicated their DNA without the cell dividing. In addition, the homologs are paired in these cells. The result is that they have a detailed pattern of bands, and rearragements such as deletions can be readily detected by studying these chromosomes.
Duplications are rearrangements in which an extra copy of part of a chromosome is present. They are referred to as "tandem duplications" if the two copies are adjacent to each other. The human genes for the pigments involved in sensing red and green color are apparent examples of tandem duplications. The proteins they code for are very similar in amino acid sequence and likely originated through tandem duplication. Duplication in general is thought to have been an important process in evolution.
Unequal crossing over can occur between these genes. This can be a cause of abnormalities leading to color blindness. Unequal crossing over is also one of the mechanism which can give rise to tandem duplications in the first place.
Inversion refers to a rearrangement which reverses gene order on a chromosome. They can be detected by loop pairing in meiotic or polytene chromosomes, or by changes in gene order on genetic maps. Two types of inversions are paracentric inversions (which do not include the centromere) and pericentric inversions (which do include the centromere). Crossing over in a paracentric inversion gives rise to a dicentric bridge and an acentric fragment. The fate of these will vary in different organisms. Crossing over in a pericentric inversion gives rise to chromosomes with duplications and deficiencies.
In general, the effect of chromosomes imbalances in the haploid stage varies between plants and animals. In plants, the gametophyte fails to function and the affected gamete is not passed on. In animals, the gamete is passed on but the offspring are likely to die or be abnormal.
Translocation refers to a rearrangement in which there is an exchange between parts of non-homologous chromosomes. Most often this results in two translocation chromosomes which have portions from different homologs. The result is four chromosomes pairing in meiosis in a quadrivalent. The effect of a translocation is an increased frequency of products of meiosis with an abnormal chromosome set. Alternate segregations from meiosis give balanced sets while two types of adjacent segregtations give unbalanced sets. The effect of these chromosome imbalances, as for inversions, is reduced fertility in plants and reduced offspring survival in animals. Robertsonian translocation is a rearrangement in which two acrocentric chromosomes join to form a metacentric or submetacentric chromosome. In humans, this can result from a joining of chromosomes 14 and 21. A high frequency of Down syndrome (trisomy 21) cases in younger mothers result from imbalances passed on my parents with this rearrangement.
Cancer is often associated with chromosome abnormalities, which related to genetic changes that have been detected in cancer. One type of genetic change involves the mutation of proto-oncogenes, normal genes involved in control of cell growth, to oncogenes. Change of only one copy of a proto-oncogene to an oncogene is needed for the cancer to form. Thus, this is a dominant mutation.