A balanced translocation is a condition in which the correct number of chromosomes are present, but two pieces of chromosomal material have switched places. The individual who has a balanced translocation usually appears completely normal and healthy, but when a carrier attempts to have children, that is when problems are encountered. Those who have a balanced translocation are plagued by infertility, repeated miscarriages, and their offspring are at an increased risk for birth defects and chromosomal abnormalities. The risk of abnormal offspring is approximately 10 to 15 percent, regardless of which parent has the balanced translocation.
What is Normal?
All human beings are made up of cells that contain our genes or DNA. Our genes are located within our chromosomes, the flexible structures that contain the blueprint for what we will look like, how we will develop and function. A normal person has 46 chromosomes, half of which come from the mother and the other half from the father. The first chromosome pairs numbered 1 through 22 are exactly the same. The 23rd pair of chromosomes determines the sex of the fetus. In a female fetus, the 23rd pair would contain two X chromosomes, while for a boy there would be one X chromosome and one Y chromosome.
What Happens with Balanced Translocation?
The egg from the mother contributes 23 chromosomes and fatherîs the sperm contributes 23 chromosomes. When fertilization occurs, the two sets join to form 23 pairs for a total of 46 chromosomes in a potential baby. When a parent has a balanced translocation, whether this ends up being a normal baby or pregnancy depends upon what chromosomes each egg or sperm contains. For example, if the mother has a balanced translocation, she has the two normal chromosomes; but she also has a copy of each that contains the translocated information from the other chromosome. If the womanîs egg contains the normal chromosomes, her offspring will be normal. If the egg has both translocated chromosomes, the baby will be normal, but will carry the same translocation as the mother. But if an egg has a good chromosome and a translocated chromosome, there will be an extra portion of one chromosome and a missing portion of the other chromosome. This would be what is termed an unbalanced translocation, which could lead to severe birth defects and an increased risk for miscarriage. In contrast to a balanced translocation, an unbalanced translocation is when there is extra or missing genetic information
Miscarriages can be caused by structural, hormonal, immunological, and environmental factors as well as genetic factors. Women that have had three or more miscarriages are candidates for genetic testing to determine if chromosomal reasons are the source of the problem. A balanced translocation is the underlying cause for two to four percent of couples with repeated miscarriages.
How is Balanced Translocation Diagnosed?
Genetic counseling or testing may be an option for those who are attempting to conceive when the woman has an advanced age, for couples that already have a child with a chromosomal abnormality, or for those that have had recurrent miscarriages. Genetic testing involves having a karotype done using blood samples from both partners. In cases of miscarriage, a test can be done using the expelled tissue. However, most of the time expelled tissue is not available, so blood from the parents is the most common method used.
A karotype is a photograph of the chromosomes from one cell of an individual normally obtained from a blood sample. The chromosomes are stained to make them visible and can only be seen when magnified 1,000 times. Chromosomes have stringy-like structures and the stain allows the bands of the chromosome to become apparent. The specialists that study the chromosomes are referred to as cytogeneticists or medical geneticists. After the analysis, the picture (karotype) is printed.
A more expensive genetic screening that is also available is Preimplantation Genetic Diagnosis (PGD). Unlike amnioscentisis, which diagnoses chromosomal and genetic abnormalities of the fetus in utero, PGD allows such identification to occur before the embryo is even in the womb. For a couple hoping to conceive that has a balanced translocation, a family history of genetic abnormalities (e.g., a child with Downîs Syndrome), or has a racial or ethnic background that make them prone to specific diseases (e.g., Sickle Cell Disease for those of African-American ancestry, or Tay Sachs for Eastern European Jews and French Canadians), this test enables them to learn the genetic make up of an embryo prior to it being placed in the womb. It can help them avoid the traumatic decision of ending a pregnancy and avoid the heartache of having another child with a fatal disease or birth defect.
PGD is used in conjunction with In Vitro Fertilization where the patient takes the required stimulation drugs. The eggs are harvested and then fertilized in a petri dish. Before any embryos are transferred to the womb, embryologists remove a cell from the embryo and determine the chromosomal and genetic composition. Those embryos with abnormalities can be identified, allowing only those that have normal compositions to be placed in the womb. A typical PGD adds between $3,500 to $4,000 to the costs of an In Vitro Fertilization cycle.
Couples who have concerns about whether they have a balanced translocation should consult their physician and discuss if genetic testing is appropriate for their circumstances. If your physician does not know of any genetic counselors, a resource to help locate one is the website of the National Society of Genetic Counselors at www.nsgc.org
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