Translocation Dr Trisha Macnair A translocation is a condition where a fragment of one chromosome is broken off and is then attached to another.

In this article

What's the cause? Balanced and unbalanced

Who's affected? How's it diagnosed?

The cause isn't understood but is sometimes linked to exposure to radiation. Depending on which bit of chromosome is moved to where, this results in a wide range of medical problems, such as leukaemia, breast cancer, schizophrenia, muscular dystrophy and Down's syndrome.

Miscarriage may occur - in five per cent of cases of recurrent miscarriage one of the parent's chromosomes is found to have a balanced translocation.

Translocations may be balanced or unbalanced. For example, imagine a translocation of part of chromosome 21 onto chromosome 14 (this is the situation in about four per cent of people with Down's syndrome). In a balanced translocation (found in the parent of an affected child) the person doesn't have any additional genetic material, they simply have a smaller than normal chromosome 21 with a piece broken off, a normal second chromosome 21, a chromosome 14 with the broken piece of 21 attached, and a normal chromosome 14. So the person appears entirely normal with no related health problems.

But when this person makes eggs or sperm there are several possibilities. The normal chromosomes 21 and 14 may be passed on, resulting in a normal baby. Or the broken 21 and affected 14 may be passed on, resulting in a baby with translocations but no overall change in the amount of genetic material - this baby would be a carrier of the balanced translocation just like its parent.

However, if the normal 21 and the affected 14 (carrying material from the broken chromosome 21) are passed on, there is now extra genetic material from chromosome 21 (as the baby will have one normal 21 from each parent as well as the broken piece attached to 14). The translocation becomes 'unbalanced' and Down's syndrome results.

There may be no symptoms from a translocation if it's balanced. Alternatively, if it's unbalanced, it may cause problems incompatible with life, leading to miscarriage of an affected foetus, or other major health problems depending on which chromosome has been translocated.

One in 500 people is thought to carry a chromosome translocation. In many cases these are balanced translocations. Unbalanced translocations may account for one in 20 cases of recurrent miscarriages.

Chromosomal analysis can rapidly reveal a translocation. Antenatal testing may be done using CVS or amniocentesis, or preimplantation genetic diagnosis when a parent is known to carry a balanced translocation.

This article was last medically reviewed by Dr Rob Hicks in December 2007