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What Is Trisomy?

Mary McMahon
Updated Mar 03, 2024
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Trisomy is a form of aneuploidy, an abnormality in which an organism has the wrong number of chromosomes. In humans, a normal baby will have 46 chromosomes in 23 pairs, with each parent contributing 23 chromosomes. When trisomy occurs, the individual is born with three instances of a particular chromosome instead of the usual two, resulting in 47 total chromosomes instead of 46. The results of this extra data can vary, but tend to manifest in the form of birth defects, some of which can be quite severe.

The most common cause of trisomy is a problem in the duplication of chromosomes to create egg and sperm cells. Somewhere along the way, a chromosome duplicates itself twice, creating a full pair. When the egg or sperm cell joins with its counterpart, the extra chromosome is taken along, creating a set of three where where should be two. In some cases, a chromosome only duplicates partially, leading to partial trisomy.

Some of the more well known forms of trisomy are trisomies 13, 18, and 21. Trisomy 13 is also known as Patau syndrome, after the doctor who identified it. Patau syndrome is characterized by physical and mental defects, with heart defects being very common. Trisomy 18 is known as Edward's syndrome, and it is accompanied by severe mental and physical problems; most patients do not survive beyond a year. Trisomy 21 is Down's syndrome, a condition which is often accompanied by severe mental disabilities.

There are other forms; trisomy 16, for example, usually triggers a miscarriage, while trisomy 9 leads to malformations in the skull and nervous system. The number in each name refers to the chromosome in question; in some cases, patients have mosaic trisomy, meaning that the defect only shows up in certain cells. Trisomy 16 is believed to be the most common form in humans, while trisomy 21 is the most common form in viable infants.

It is possible to test for trisomy, along with other potential birth defects, and women who are at risk for the condition may undergo prenatal testing to check for it. If a prenatal 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 may choose to carry on with the pregnancy anyway, for ethical or moral reasons. Some people with trisomy leave happy, full lives, while other infants survive for only a short time after birth.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a TheHealthBoard researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon283638 — On Aug 05, 2012

My nephew was just told that his premature infant has low set ears, droopy eyes, underdeveloped kidneys and inflamed lungs. This was most of the information I was given. He and his wife were also told that survival in infants with this condition, mostly in Caucasian males and African American females, is usually no more than two to two and a half months.

They didn't mention the name of the condition, but Trisomy sure sounds close. If someone could give me any feedback, I would appreciate it. I'm just trying to understand what might be going on so I can be more supportive.

By ElizaBennett — On May 21, 2011

Sadly, dfoster85 is right about trisomy 13 and 18, but there is actually another trisomy that people can live with: trisomy x (triple x syndrome). A woman I work with was terrified when her amnio showed three x chromosomes, but apparently most people with the condition are quite healthy. It's not like Turner Syndrome, which is really obvious--her daughter is four now and you would never know to look at her that she was "different." She will probably develop quite normally and be able to have children of her own one day.

By dfoster85 — On May 20, 2011

I want to clarify the comment at the end. People with Down Syndrome (Trisomy 21) can lead full lives, as can some people with trisomy mosaicism. But the other trisomies rarely result in live births and when they do, life tends to be quite short for those individuals, especially boys.

The longest-lived person with trisomy 18 was a woman who lived into her forties, but even making it into the teens or twenties is considered unusual. And babies born with trisomy 13 have even shorter lives. Despite the few exceptions, the medical community (according to my sister the nurse) considers these conditions "incompatible with life."

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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