What causes Mosaic trisomy 9 ?
What causes mosaic trisomy 9? Most cases of mosaic trisomy 9 occur due to a random event during the formation of the reproductive cells (egg and sperm) or after fertilization has taken place. An error in cell division (called nondisjunction) may cause some eggs or sperm to have an abnormal number of chromosomes. If an egg or sperm with an extra chromosome 9 contributes to the genetic makeup of an embryo, the embryo will have an extra copy of chromosome 9 in each cell. As the embryo grows and divides, an attempt may be made to correct the mistake by eliminating one extra chromosome 9. In people with mosaic trisomy 9, this attempt may be partly successful, leaving some cells with an extra chromosome 9 and some cells with the extra chromosome deleted (the usual chromosome number). This correction process is called trisomy rescue. In other cases, the egg and sperm may have a normal number of chromosomes, but an error of cell division (nondisjunction) occurs when the fertilized egg is growing and dividing. If an error occurs during one of the divisions, it can cause some cells to have an abnormal number of chromosomes. In people affected by mosaic trisomy 9, some of the body's cells have the usual two copies of chromosome 9, and other cells have three copies of this chromosome (trisomy). The percentage of cells with trisomy 9 and which parts of the body are affected vary from person to person. This leads to variability in the range and severity of symptoms. In rare cases, mosaic trisomy 9 is inherited from a parent with a chromosomal rearrangement called a "pericentric inversion." This occurs when a segment of chromosome 9 has broken off in two places, swiveled round 180 degrees and reinserted itself into the chromosome. If this rearrangement is considered "balanced," meaning the piece of chromosome is in a different order but no genetic material is gained or lost, it usually does not cause any symptoms or health problems. However, it can be associated with an increased risk of having children with an abnormal number or chromosomes.