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Reciprocal Translocation ( between the arms of any two chromosomes )


What is a chromosome disorder and what is its significance?

Chromosomes are the structures in which our hereditary material (DNA) is packed. A healthy person has 46 chromosomes (23 pairs). One chromosome of each pair (i.e. a total of 23 chromosomes) is donated by each parent to the fetus. Twenty-two pairs are called autosomes, and the 23rd pair consists of the sex chromosomes (X and Y). Females have two X chromosomes and males have one X and one Y. The chromosome pairs are arranged and numbered by size and shape, and each has different genes. Because there are two copies of each chromosome, there is effectively a duplicate gene system (one maternal and the other paternal), each identical in sequence to the other.

A change in the number or structure of the chromosomes in pairs 1-- 22, causing an addition or deletion of chromosomal information in these pairs compared with the normal state, constitutes a significant medical problem. In the great majority of cases, this change is expressed as severe structural or functional defects in the tissues and/or mental retardation. For example, an extra number 21 chromosome, resulting in three instead of the normal two so that there are 47 chromosomes altogether, causes Down syndrome. An extra number 18 chromosome, which is larger and therefore contains more genes, manifests as a more severe disorder called trisomy 18. In this syndrome, there are severe defects, and it usually ends in death soon after birth or even in the uterus. This is also the case with a deletion of the whole or a part, however small, of chromosome 18.

Conversely, the clinical manifestation of a change in the number of sex chromosomes (X or Y) or their structure is usually less severe. See under "chromosomes".

What is a translocation?



This is the exchange of chromosomal material between two chromosomes of different pairs.

A translocation is said to be balanced when a change occurs in the location of certain genes, but there is no gain or loss of genetic material.

If a person carries a balanced translocation, with no change in the amount of genetic material and no damage to the genes has occurred, he will be healthy.

A parent carrying a balanced translocation can have healthy children without a translocation, or healthy children carrying a balanced translocation like that of the parent.
In these cases, where the chromosome arrangement is identical to that of the affected parent, there are no clinical manifestations. The probability that a parent who carries a balanced translocation will have healthy children is approximately 85% per pregnancy. However, such a parent does have an increased risk (approximately 15%) of having a child with an unbalanced chromosome arrangement. In such a child, the chromosome arrangement will include gains or losses of genetic material that result from the parental translocation. Some fetuses that have an unbalanced chromosome arrangement abort spontaneously at the beginning of pregnancy, so that some of the couples where one partner carries a balanced translocation have recurrent miscarriages. This can be diagnosed during pregnancy by amniocentesis or chorionic villus sampling.

There are two main types of translocations:

  1. Reciprocal translocation - described below. An inversion is also effectively a translocation of this type.

  2. Robertsonian translocation - see corresponding information sheet.

Reciprocal translocation (and/or inversion)

This is a condition in which there is an exchange of chromosomal material between two chromosomes of different pairs. In this condition, there is breaking and rejoining of two arms of two different chromosomes (reciprocal translocation) or two arms of the same chromosome (inversion). When the break occurs between the genes and the genes are not damaged, there are no clinical consequences. However, when as a result of such a translocation, damage is caused to an essential gene, a mutation that can cause impaired functioning may arise.

Reciprocal Translocation
The significance depends firstly on whether or not one of the parents carries the same translocation (or inversion) - it is unlikely that both have it.

If one of the parents does carry the translocation, it may also be present in other family members.

In most cases, one of the parents carries a similar translocation.

If one of the parents carries such a translocation (or inversion)

If the parent carrying the translocation is completely healthy, it can be expected that the fetus will also be healthy, without genetic problems arising from the translocation.

If neither parent is a carrier of such a translocation (or inversion)

The chance of finding such a translocation in the fetus on performing amniocentesis in pregnancies of couples where neither is a carrier of this translocation is 1 in 2,000. If the translocation is not present in either parent, this is described as a de novo translocation in the fetus, and in this situation the risk for defects or developmental problems and mental retardation in the baby is 6% - 7%. The defects are diverse and some can be detected by ultrasound. Therefore, if defects are not detected by ultrasound, the likelihood that there will be problems in the newborn is lower (see below). Conversely, if defects or growth retardation are found on ultrasound examination, the risk that the child will be mentally retarded is significantly increased. In genetic counseling, the risk can be assessed based on the ultrasound findings.

The most common inversions usually occur in chromosomes 2 and 9 and do not involve a significantly increased risk for defects or mental retardation. These inversions are usually present in one of the parents too. When a fetus carries an inversion, the chance that this is present in both parents is very low; 1 out of 10,000 amniocenteses. In these cases, the inversions are usually not in the common chromosomes.

Are cases of de novo translocation (or inversion) associated with an increased risk for mental retardation if the ultrasound scan is normal?

Based on a number of reports in which a total of approximately 150 cases were studied, half of the fetuses carrying translocations had only mental retardation and no defects. It thus appears that the absence of defects demonstrated by ultrasound is associated with a greater chance that the fetus will be healthy, but there is still at least a 3% - 4% risk for mental retardation (twice the normal risk).

It should be mentioned that to date not enough studies have been carried out that have monitored enough children with de novo translocations detected in pregnancy for long enough to draw conclusions on their development.
 
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Comments (9)


Monday, May 31, 2010 10:41 AM
(1) toboso  says:


Oncogenesis

i would like to ask ,what is the role of reciprocal translocation in oncogenesis?


Tuesday, June 01, 2010 11:27 PM
(2) Sir-me  says:


oncogenesis

where do we find the solution to the role of reciprocal translocation in oncogenesis if not here?


Tuesday, June 01, 2010 11:54 PM
(3) Sir-me!  says:


oncogenesis

nkt!Runo is sick or what?


Wednesday, June 02, 2010 6:38 AM
(4) NjaBe  says:


oncogenesis

i would also like o know what the role of reciprocal translocation in oncogenesis is...


Thursday, June 03, 2010 10:54 PM
(5) ndalo victor  says:


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Friday, June 04, 2010 3:48 AM
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Friday, April 08, 2011 8:07 AM
(7) kelcie  says:


confused

i have had 3 misscarrages.. 1 when i was 16,, and 2 within the last 2 years.. my doctor did a chromosone test and come to find i have reciprocal translocation between chromosomes 8 and 18.. i just want to know what my chances of having a normal healthy baby if any,, please and thank you i would be very greatful to anyone with an answer. kelcie


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