The mostly seen type of the disease is the one seen once at every 500 births called balanced translocation (reciprocal or robertson translocation). If the chromosome group has normal complement of genetic knowledge, the structural re-arrangements are called balanced. Some arrangements have stability and they are conveyed without any change during cell division. To be able to accept a chromosome which is re-arranged as stabile, The chromosome have to has normal structure (only one centromere or two telomerases).

TRANSLOCATION

If a piece is broken off from chromosome and it locates another chromosome, a translocation event occurs. Translocations are divided into three subtitles:

• Reciprocal translocation
• Rentric translocation ( robertsonian )
• Insertional translocation (transposition)

Reciprocal translocation

It is a break off two chromosomes and  reciprocally replacements of their pieces. It is observed at the rate of 1/500.

The mechanism of reciprocal translocations



This rearrangement does not make a phenotype effect. But because of unbalanced gamete, at every pregnancy there is high risk (85%).

Probable embryos at the disease carrier of reciprocal translocations




Centric Translocation ( Robertsonian)

That observed between two homological or non-homological 2 acrecentric chromosomes is called centric translocation. On this rearrangement long and short arms of two different chromosomes merged with themselves. United short arms disappear and just two different long arms merged their centromeric areas are observed. These disappeared chromosome pieces of short arms do not make an effect because of having multiple copies of rRNA genes just like at acrocentric chromosomes. At this patients have 45 chromosomes. Even if there a quantitative deficiency, there is no change on genetic material.

Robertsonian translocation mechanism



At peoples having Robertsonian translocations, there is a high possibility to see gamete cells having unbalanced genetic structure even if it has lower percentage (66%) than that at reciprocal translocations.

Probable embryos at Robertsonian translocations




Insertion translocation (Transpozition)

While one chromosome is broken off from two different points, another chromosome is broken off at one point. After these breaks off,  one piece of double broken chromosomes goes into the other chromosome and it is united with it. The children of these people, it could be observed duplications or deletions of the piece inserted the other chromosome because of segregation anomalies.

Insertion translocation mechanism




DELETIONS (DECREASE)

A chromosome segment disappears after it is broken off and this causes an unbalance. This  breaking can be two different ways at one of which is terminal piece disappears (terminal deletion) and at the other one is that after double breaks, extracted piece disappears and chromosome is again united. Extracted piece disappears because it is not centromeric generally.  But if it is centoromeric, by holding spindal fibers it goes to the polars and does not disappear during cell division. This situation has no repetition risk.

Deletion mechanism




DUPLICATION (INCREASE)

At one of two homologous chromosomes, there occurs double breaks, while another chromosome is broken off once. After these events the piece that is broken off from double broken chromosome goes into the other chromosome and it is inserted on it. Duplication is mainly observed at meiosis. And there are double copies of a piece of a chromosome.

Duplications mechanism




INVERSION

There are two breaks off from two different points of a chromosome. This extracted piece is united at same place after being reversed in itself. This inversion in general does not cause a  phenotype at disease carriers because there is a balanced arrangement. There are two types of inversion:

• Paracentric inversion
• Pericentric inversion

Paracentric inversion

There is a break off, not including centomere, at either long or short arms. After the breaking, the piece is united again in the form of reverse. Because of the fact that there is no chance on height, it can be defined by the band structure method. But gene line changes and it does not affect phenotype. Furthermore in the gamete dispersion of people who have inversions, recombinant chromosomes that generally occur at unbalanced gametes are in the form of acentric or dicentric. Even if this type of inversion is reported very rarely, they do not let give birth alive. Hence the risk of having a child alive is very low for a disease carrier.

Paracentric inversions mechanism



Pericentric inversion

There are two breakings, including centomere, at both long and short arms. After the breaking, the piece is merged again in the form of reverse. Both the height of the chromosome  and the gene line are changed. An individual having this type of inversion produces gametes having duplication or deficiency of chromosome segments. These segments are pieces stayed inversion distal. The risk of having a child with unbalanced karyotype is 1%-15%. And pericentric inversion has its specific risks. At people having large paracentric inversion, the pieces left at distal are less than those at small pericentric inversion. And also it is observed that there are decrease or increase on pieces left at distal. Therefore having recombinant children for these people is more possible. Inversion 9 is the best known inversion seen human chromosomes. Because of the fact that on studies it is transcribed that this inversion does not cause anomaly births, it is accepted as polymorphism.

Pericentric inversions mechanism