Mosaic trisomy 9
What is mosaic trisomy 9?
We each have 46 chromosomes that are separated into 23 pairs. One copy in each pair comes from our mother and one copy in each pair comes from our father. The first 22 pairs of chromosomes are numbered 1 through 22 and are arranged from the largest chromosomes to the smallest chromosomes. The last pair of chromosomes determines our gender. Women have two X chromosomes while men have one X chromosome and one Y chromosome. In complete trisomy 9, all of the cells of the body have an extra chromosome 9, which means that there are 47 chromosomes in all the cells instead of 46. A baby with complete trisomy 9 will usually not survive to birth. In mosaic trisomy 9, some of the cells of the body have an extra chromosome 9, which means that there are 47 chromosomes in those cells and other cells in the body have the typical 46 chromosomes. The cells with the extra chromosome 9 are called "trisomic" as they have three copies of the 9th chromosome.
Having mosaic trisomy 9 causes a range of symptoms. Common features of mosaic trisomy 9 include growth problems before birth (intrauterine growth retardation IUGR), intellectual disability, heart differences that are present at birth (congenital heart defects), and differences of the skull and facial (craniofacial) region, such as a sloping forehead, a bulbous nose, short eyelid folds, deeply set eyes, and low-set, unusually shaped ears. Individuals may also have skeletal, genital, and kidney (renal) problems.
When mosaic trisomy 9 is found before a baby is born, through amniocentesis or chorionic villus sampling, it can be difficult to predict what health problems will occur. When mosaic trisomy 9 is found by chorionic villus sampling there is a chance that the cells with trisomy 9 are only found in the placenta and not in the baby (confined placenta mosaicism). The risk for confined placental mosacism is approximately 1%. In this case, an amniocentesis may be suggested to look at the baby's cells directly.
When the majority of a baby's cells have trisomy 9 they may be sick and pass away early in life. Individuals with fewer cells that have the extra chromosome 9 may have less health issues; however, it can be difficult to predict severity of symptoms based on the amount of cells with trisomy 9 since we are unable to test all cells in the body to determine which cells have 47 chromosomes and which cells have the typical 46 chromosomes.
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