In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translocation is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes. Two detached fragments of two different chromosomes are switched. Robertsonian translocation occurs when two non-homologous chromosomes get attached, meaning that given two healthy pairs of chromosomes, one of each pair "sticks" together. A gene fusion may be created when the translocation joins two otherwise-separated genes. It is detected on cytogenetics or a karyotype of affected cells. Translocations can be balanced or unbalanced.
Reciprocal translocations
Reciprocal translocations are usually an exchange of material between non-homologous chromosomes and occur in about 1 in 491 live births. Such translocations are usually harmless and may be found through prenatal diagnosis. However, carriers of balanced reciprocal translocations have increased risks of creating gametes with unbalanced chromosome translocations, leading to Infertility, miscarriages or children with abnormalities. Genetic counseling and genetic testing are often offered to families that may carry a translocation. Most balanced translocation carriers are healthy and do not have any symptoms. It is important to distinguish between chromosomal translocations occurring in gametogenesis, due to errors in meiosis, and translocations that occur in cellular division of somatic cells, due to errors in mitosis. The former results in a chromosomal abnormality featured in all cells of the offspring, as in translocation carriers. Somatic translocations, on the other hand, result in abnormalities featured only in the affected cell line, as in chronic myelogenous leukemia with the Philadelphia chromosome translocation.
Nonreciprocal translocation
Nonreciprocal translocation involves the one-way transfer of genes from one chromosome to another nonhomologous chromosome.
is a type of translocation caused by breaks at or near the centromeres of two acrocentric chromosomes. The reciprocal exchange of parts gives rise to one large metacentric chromosome and one extremely small chromosome that may be lost from the organism with little effect because it contains few genes. The resulting karyotype in humans leaves only 45 chromosomes, since two chromosomes have fused together. This has no direct effect on the phenotype, since the only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number. Robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. The most common translocation in humans involves chromosomes 13 and 14 and is seen in about 0.97 / 1000 newborns. Carriers of Robertsonian translocations are not associated with any phenotypic abnormalities, but there is a risk of unbalanced gametes that lead to miscarriages or abnormal offspring. For example, carriers of Robertsonian translocations involving chromosome 21 have a higher risk of having a child with Down syndrome. This is known as a 'translocation Downs'. This is due to a mis-segregation during gametogenesis. The mother has a higher risk of transmission than the father. Robertsonian translocations involving chromosome 14 also carry a slight risk of uniparental disomy 14 due to trisomy rescue.
Cancer: Several forms of cancer are caused by acquired translocations ; this has been described mainly in leukemia. Translocations have also been described in solid malignancies such as Ewing's sarcoma.
Infertility: One of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived fetuses are not viable.
Down syndrome is caused in a minority of cases by a Robertsonian translocation of the chromosome 21 long arm onto the long arm of chromosome 14.
Chromosomal translocations between the sex chromosomes can also result in a number of genetic conditions, such as
XX male syndrome: caused by a translocation of the SRY gene from the Y to the X chromosome
By chromosome
Denotation
The International System for Human Cytogenetic Nomenclature is used to denote a translocation between chromosomes. The designation t is used to denote a translocation between chromosome A and chromosome B. The information in the second set of parentheses, when given, gives the precise location within the chromosome for chromosomes A and B respectively—with p indicating the short arm of the chromosome, q indicating the long arm, and the numbers after p or q refers to regions, bands and subbands seen when staining the chromosome with a staining dye. See also the definition of a genetic locus. The translocation is the mechanism that can cause a gene to move from one linkage group to another.
Examples
History
In 1938, Karl Sax, at the Harvard University Biological Laboratories, published a paper entitled "Chromosome Aberrations Induced by X-rays", which demonstrated that radiation could induce major genetic changes by affecting chromosomal translocations. The paper is thought to mark the beginning of the field of radiation cytology, and led him to be called "the father of radiation cytology".
