Chromosomal translocation

Under a translocation ( locomotion, transfer, locus of Latin: place ) is understood in genetics, a chromosome mutation, were transferred to the chromosome segments to a different position within the chromosome population. In extreme cases, an entire chromosome can attach to another.

With the exception of Robertson translocation, which is abbreviated to "rob " the karyotype is a translocation with "t " hereinafter.

Forms of translocation

Balanced translocation

In a balanced translocation ( = balanced in equilibrium ) is a chromosome or a chromosome segment translocated to another chromosome, where the total amount of genetic material does not change but remains in balance. Because of a balanced translocation has phenotypically no effect on the person concerned. However, people with a balanced translocation have an increased likelihood of an unbalanced translocation to produce offspring (see below), because they also form germ cells ( gametes) with an unbalanced translocation. Find a fertilization between a germ cell instead of an unbalanced translocation and a germ cell in which the genetic material is arranged as usual, the growing child has a translocation trisomy. Pregnant women with a balanced translocation are at an increased risk of losing her unborn child by miscarriage. Clinical significance can be a balanced translocation also in carcinogenesis. In acute myeloid leukemia ( AML) is found in approximately 20% of cases a balanced translocation in hematopoietic progenitor cells. Fusion genes may result, which play an important role in the activation of genes for proliferation and differentiation.

The karyotype of the existence of a balanced translocation can for example be: 45, XX, t (14, 21 ) or 45, XY, t (14, 21 ) (if a whole chromosome is translocated ) or 46, XX, t (14, 21 ) and 46, XY, t (14; 21) ( provided that it is only a part of the translocated chromosome ).

Unbalanced translocation

An unbalanced translocation is characterized by a quantitative change in the genome, which is caused by the fact that genetic material is lost or in addition to the normally existing genetic material is present. An example of this form of translocation, the translocation trisomy 21, in which extra genetic material has the chromosome 21 attached to another chromosome. The karyotype of a translocation, for example, trisomy 21 are as follows: 46 21 XX, t (14, 21 ) or 46, XY 21, t (14, 21 ) / translocation trisomy 21 / 14th

Reciprocal translocation

In a reciprocal translocation (Latin: reciprocus: mutually, each other ) a piece of exchange between non-homologous chromosomes has taken place. Again, the Genmenge remains in balance. Is important to note that it arrives here on the segregation of chromosomes in the gametes: Let's call the "normal" chromosomes N1 and N2, the " translocated " T1 and T2. In so-called Adjacent - 1 segregation comes along N1 N2 T2 and T1. This combination is usually fatal. The other option is the alternative segregation, N1 N2 and T1 T2, wherein the combination is produced, two complete and viable gametes. This whole process produces gametes in a 1:1 ratio. Half of the gametes is not viable, what is referred to as a semi- sterility.

Burkitt 's lymphoma is characterized by a reciprocal translocation between chromosome 8 and 14, mantle cell lymphoma by a similar between chromosomes 11 and 14.

Robertson's translocation ( centric fusion)

Robertson's translocation ( Robertsonian translocation or even derivative chromosome in karyotype abbreviated to " rob" or " the " ) is a special type of translocation, which occurs relatively frequently ( very often between chromosomes 13 and 14) and for the first time in year was described in 1916 by WRB Robertson and due to its named after him.

It is caused by the displacement of a relatively large chromosomal segment to a non - homologous chromosome: It connect two acrocentric chromosomes ( chromosomes whose waists are located near the ends of chromosomes ) to a metacentric chromosome (chromosome whose constriction in the middle is ). This is done through a merger ( connection, fusion ) of two long arms acrocentric chromosomes in the centromeric region ( = the area of ​​the constriction where the spindle fibers in the division schedule ), the two short arms are lost (see deletion ) and thus a metazentrisches chromosome is produced.

This form of translocation can be demonstrated in humans at the acrocentric chromosomes of the number 13, 14, 15, 21, and 22, in which the centromere is in each case close to the end (as opposed to metacentric chromosomes in which the centromere is created right in the middle ). The loss of the short arms in a merger of the two long arms falls in acrocentric chromosomes not particularly significant since the short arms contain no relevant genes.

A Robertson's translocation may be balanced or unbalanced. People with a balanced translocation Robertson have the usual phenotype. In the representation in the karyotype but their genotype falls on the fact that 45 chromosomes can be detected instead of the usual 46. Although you can offspring with an equally inconspicuous phenotype, but it comes at the gametogenesis heaped for the production of gametes with quantitatively and qualitatively different sets of chromosomes: The fused chromosome or a missing chromosome or an extra long arm of the acrocentric chromosome can be passed on to offspring be. The karyotype in the presence of a balanced translocation Robertson, for example, are as follows: 45, XX, rob (14, 21 ) or 45, XY, rob (14, 21 ), the karyotype in the presence of an unbalanced translocation Robertson for as: 46, XX 21, rob (14, 21 ) and 46, XY, 21, rob (14, 21 ). In the latter case the person concerned has Down syndrome with Robertson's translocation 21/14 ( Translokationstrisomie 21).

In mice, it is known for some time to the frequent occurrence of Robertson translocations. There are known 124 of 171 possible forms of Robertson's translocation.

Philadelphia chromosome

The so-called Philadelphia chromosome has a transfer ( transfer, transferre from Latin: transfer ) of the main part of the long arm of chromosome 9 occurred after chromosome 22. Through a reciprocal translocation gene Bcr melts on chromosome 22 with the gene Abl on chromosome 9 together and produce the protein Bcr -Abl. This results in a malfunctioning gene for the Abl kinase. This change occurs in over 95 % of people with chronic myeloid leukemia and 5 % of children and 20 to 30 % of adults with acute lymphoblastic leukemia before.

Examples of Evolution

By comparison with the great apes ( chimpanzees, gorillas and orang- utans, each with 48 chromosomes) translocation was localized gorilla - specific. The reconstruction is either the original karyotype of the group with 48 chromosomes, as it now exists in chimpanzees and orangutans. The chromosomes involved correspond to human chromosomes 5 and 17, but the part with only 46 chromosomes also to the superfamily of human -like ( Hominoidea ).