Trisomy Awareness Month · Trisomy 15: Prader-Willi Syndrome · Angelman Syndrome · Trisomy.org
Trisomy is an abnormality in which an organism has the wrong number of chromosomes.
A normal human will have 46 chromosomes in 23 pairs, with each parent contributing 23 chromosomes. When trisomy occurs, a child is born with three instances of a particular chromosome instead of the usual two, resulting in 47 total chromosomes. In some cases, a chromosome only duplicates partially, leading to partial or mosaic trisomy. While some people with trisomy lead happy, full lives, the outcome tends to be birth defects, some of which can be quite severe.
The number of each trisomy refers to the chromosome affected. The most common trisomies: Trisomy 21 (Down Syndrome), a condition which is often accompanied by mild to severe mental disabilities and the most common form in viable infants; Trisomy 13 (Patau Syndrome), characterized by physical and mental defects, with heart defects being very common; and Trisomy 18 (Edwards Syndrome), accompanied by severe mental and physical problems (most patients do not survive beyond a year).
Trisomy 16 is believed to be the most common cause of miscarriage, while Trisomy 9 leads to malformations in the skull and nervous system. There are two forms of Trisomy 15: though both feature multiple developmental delays, an extra 15th chromosome from the mother results in Prader-Willi Syndrome (PWS), which can result an uncontrollable appetite; and an extra copy of the father’s 15th chromosome results in Angelman Syndrome (AS), involving behavioral, social skill, and mobility issues.
It is possible to test for trisomy, along with other potential birth defects, and women who are at risk for the condition can undergo prenatal testing. If a test such as an amniocentesis reveals a trisomy, a doctor will discuss options with the patient. In some cases, doctors may recommend that the pregnancy be terminated, because the baby is nonviable. Some parents carry on with trisomy pregnancies and births for ethical or moral reasons.