Chromosome translocation is described in genetics as an abnormality of the chromosome structure. This abnormality in the chromosome structure is caused by breaking off of a fragment from one chromosome and it moving forward to attach with another. A rearrangement of parts takes place within the no homologous chromosomes. This feature can be detected through cytogenesis, or karyotype of affected cells.

Chromosomal Translocation Causes

Though the real cause behind such disarrangement in the chromosomes has not yet been properly detected, one of the main causes is believed to be an exposure to radiation for a prolonged time period.

Chromosomal Translocation Types

Chromosomal translocation can be

Non-Robertsonian (Reciprocal)

Robertsonian

Balanced

Unbalanced

Reciprocal (non-Robertsonian) translocations

Usually an exchange of material between nonhomologous chromosomes occurs in non-Robertsonian translocation. In about 1 in 600 human newborns, such chromosomal translocation is found.

They are usually harmless and are generally found out in prenatal diagnosis. Carriers of balanced reciprocal translocations display more risks of creating gametes with unbalanced chromosome translocations that might lead to either miscarriages, or to the birth of children with abnormalities. Families with a history of chromosomal translocation need genetic counseling and genetic testing.

Robertsonian translocations

Two acrocentric chromosomes are involved in this type of translocation. These two chromosomes fuse near the centromere region. This results in the loss of the shorter arm. Since two chromosomes fuse together, the resulting karyotype in human beings leave only 45 chromosomes. All combinations of acrocentric chromosomes are involved in Robertsonian translocations.

Chromosomes 13 and 14 are involved in most of the common chromosomal translocation in human beings. This type is seen in 1 of 1300 human beings. The carriers of Robertsonian translocations, like all other chromosomal translocations, are phenotypically normal.

However the risk of unbalanced gametes leading to miscarriages or abnormal babies still remain. For example, Robertsonian translocations involving chromosome 21 has a higher chance of leading to a baby with Down syndrome.

Balanced and unbalanced translocation

Translocations might either be balanced or unbalanced. For example, in most of the people suffering from Down syndrome, there is a translocation of part of chromosome 21 into chromosome 14.

In a balanced translocation that is generally seen in the parent of the child affected with such disfunctioning no additional genetic material can be seen; rather simply a smaller chromosome 21 with a part of it missing is noticed. This makes the person appear entirely normal without any related health problems.

But there are several possibilities of abnormality when this affected person, who otherwise seems to be perfectly ok, starts producing eggs or sperm. If the normal chromosomes 21 and 14 were passed on, it would result in a normal baby. If the broken chromosome 21 and affected chromosome 14 is passed on, it will result in a baby with chromosomal translocations but no abnormality in the genetic material.

This baby would be a carrier of the balanced translocation like its parent, but not a patient of it. However if normal 21 and affected 14 (carrying materials from broken chromosome 21 are passed , the translocation becomes unbalanced resulting in Down's syndrome.

What are the diseases caused by chromosomal translocation in humans?

The cause of chromosomal translocation is not clear but is sometimes linked to exposure to radiation. Depending on the movement of the broken parts of the chromosomes, the diseases can be widely diversified as follows:

Recurring miscarriages

Leukaemia (acute myelogenous leukemia and chronic myelogenous leukemia).

Infertility

Breast cancer

Schizophrenia,

Muscular dystrophy

Down's syndrome.

Target patients of Chromosomal Translocation

There are innumerable cases of balanced chromosomal translocations. About one in 500 cases are like this. It is the Unbalanced translocation that is dangerous, occurring in about 1 in 20 cases of miscarriages.

Diagnosis

Chromosomal analysis— This rapidly reveals a translocation.

Antenatal testing—This is done using CVS

Preimplantation genetic diagnosis—In cases when a parent is already known to carry a balanced translocation.

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