Translocation

Form of chromosome mutation in which parts of chromosomes have been exchanged both within a chromosome and with another chromosome. As with other mutations, this changes the structure of the chromosome. The result can be an altered sequence of genes with specific consequences.

• In a balanced translocation , chromosomes or a part of them are transferred to another chromosome. This translocation is balanced if the total number of genes is unchanged and no gene is damaged at the chromosome break sites. This mutation does not affect the phenotypic characteristics of the organism.
• In an unbalanced transl ocation, the loss or addition of a chromosome or a fragment alters the amount of genetic material. An example of this is translocation trisomy 21. Inheritance of translocation chromosomes can result in a genome with supernumerary or missing chromosome parts (partial trisomies or monosomies ). This mutation can cause numerous phenotypic abnormalities and malformations.
• In reciprocal translocation , there is a mutual exchange of non-homologous chromosomes, with the total number of genes remaining the same. The translocation is balanced (e.g. Philadelphia chromosome).
• In centric fusion or " Robertson translocation", two so-called acrocentric chromosomes join together to form a novel, larger chromosome. This mutation can be balanced or unbalanced. In this translocation, a centromere and the short arms of the chromosomes involved (these are without genetic information) are lost (see figure). Acrocentric chromosomes are chromosomes 13, 14, 15, 21, 22. These chromosomes can fuse centrically with each other.

• The Philadelphia chromosome is a reciprocal translocation t(9:22) caused by a translocation of the major part of the long arm of chromosome 9 to chromosome 22. The Philadelphia chromosome is detected in > 95% of patients with chronic myeloid leukemia and in 5% of all children and 20 to 30% of adults with acute lymphocytic leukemia.
• In Burkitt's lymphoma a reciprocal translocation between chromosomes 8 and 14 is observed (t(8:14)).
• In mantle cell lymphoma a reciprocal translocation between chromosomes 11 and 14 is observed (t(11:14)).

In genetics, a short form is used to define a translocation exactly. For example, a translocation (t) 46,XY,t(2;6)(q21;p13) would be interpreted as follows: Band 1 in region 2 of the long arm of chromosome 2 has been exchanged with band 3 on region 1 of chromosome 6.