Balanced Translocation

We are all made up of cells that contain our genetic information, or DNA, which determines at conception how we will develop, how our bodies will function and what we will look like. Compact spools of DNA are called chromosomes, which enable our genes to fit into the nucleus of each of our cells. Normally, we have 46 chromosomes in each cell; 23 from our mothers and 23 from our fathers. When fertilization occurs, the two sets of chromosomes join together to form 23 pairs for a total of 46 chromosomes. If a person has the correct number of chromosomes, but their pieces are joined up incorrectly, he or she has what is called a balanced translocation.

When a person has a balanced translocation, his or her chances of having a healthy pregnancy and baby depend on which chromosomes each egg or sperm contains. For example, if the woman has a balanced translocation, she has the two normal chromosomes, as well as a copy of each that contain the translocated information. If a woman's egg contains the normal chromosomes, her baby will be normal. If the egg has both translocated chromosomes, the baby will be normal, but will have the same translocation as the mother.

One in 500 people are thought to carry a chromosome translocation, but there may be no symptoms if it is a balanced translocation. However, it can also be unbalanced. An unbalanced translocation occurs when there is extra or missing genetic information. If a woman's egg, for example, has one normal chromosome and one translocated chromosome, it will result in an extra part of one chromosome and a missing part of the other. An unbalanced translocation such as this can lead to recurrent miscarriages and severe birth defects.

To better understand the difference between balanced and unbalanced translocation, imagine a part of chromosome 21 has attached itself to chromosome 14. In a balanced translocation, the person has 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. The person appears completely normal and suffers no related health problems. If this person wants to have children, however, there are several possibilities that could affect the outcome of the pregnancy.

• Both normal chromosomes - The normal chromosomes 21 and 14 may be passed on, resulting in a normal baby.
• Broken 21, affected 14 - The broken chromosome 21 and affected chromosome 14 (with the broken piece from chromosome 21 attached) may be passed on, resulting in a baby with the same balanced translocation as the parent, but no related.
• Normal 21, affected 14 - The normal chromosome 21 and affected chromosome 14 may be passed on, giving the child extra DNA from chromosome 21, resulting in unbalanced translocation and Down's Syndrome.

The cause of translocation is not well understood, though it has been linked to radiation exposure. There are a wide range of medical problems that may result from the condition, including leukemia, breast cancer, schizophrenia, muscular dystrophy and Down's Syndrome. The related health problems suffered by a person with translocation depend on which part of which chromosome is moved to where.

Recurrent miscarriages can also result from balanced translocation. Women who have had three or more miscarriages should undergo genetic testing to see if chromosomal abnormalities are the source of the problem, as unbalanced translocations may account for one in 20 cases of recurrent miscarriages.

When a chromosome translocation is suspected, genetic testing is done by drawing blood samples from both partners and the chromosomes are stained to make them visible when magnified 1,000 times. A photograph of the chromosomes called a karyotype is then printed and analyzed by a cytogeneticist.

There is another, more expensive type of genetic screening called Preimplantation Genetic Diagnosis (PGD). While amniocentesis can only diagnose chromosomal and genetic abnormalities in an actual fetus, PGD can detect these problems in an embryo before it is even in the womb. Used with in vitro fertilization (IVF) treatment, PGD allows doctors to identify embryos with abnormalities outside of the body so that only the normal embryos will be placed in the uterus. Typically, PGD costs anywhere between $3,000 and $4,000, in addition to the cost of IVF. Although it is expensive, this test can help couples with balanced translocation or a family history of genetic abnormalities avoid the decision to end a pregnancy or have a child with severe birth defects.