What Are the Different Types of Trisomy?

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  • Originally Written By: Drue Tibbits
  • Revised By: C. Mitchell
  • Edited By: PJP Schroeder
  • Last Modified Date: 15 January 2019
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The family of genetic conditions known as “trisomies” happen when certain cells have three, rather then two, chromosomes. Trisomies can happen on any one of the human body’s 23 chromosomes, and are usually named by number according to the chromosome to which they’re attached. The most common mutation affects the 21st chromosome, and is usually diagnosed as Down syndrome. Edwards syndrome impacts the 18th chromosome, and Patau’s syndrome concerns the 13th. Some conditions, known as full trisomies, impact nearly every chromosome, while so-called “mosaic” conditions affect just a smattering. All of these typically present with physical and mental symptoms that can range from mild to severe and even life threatening. Most of the time there isn’t anything expectant parents can do prevent their child from developing one of these conditions. Advancing maternal age makes occurrence more likely and taking prenatal vitamins before conception may reduce risk, but nothing is foolproof.

Understanding Trisomies Generally

Trisomy is a genetic condition in which cells have an extra chromosome. Normally, each cell in the human body carries 23 pairs of chromosomes. At conception, when cells begin to divide, an extra chromosome may attach to a pair of chromosomes. This creates cells with 47 chromosomes rather than 46. The extra chromosome is usually expressed with distinctive physical and mental characteristics.


Down Syndrome

Trisomies are typically numbered according to the location of the extra chromosome in relation to the 23 present in the chain. Trisomy 21, also called Down syndrome, is the most common mutation and happens when there are three chromosomes in the 21st location. Down syndrome typically manifests physically as well as mentally, with those affected having wider faces, distinguishable features, and low muscle tone in addition to profoundly limited intellect. Heart and organ problems are common as well, but many if not most people with this extra chromosome do survive to adulthood. They typically need a lot of care for life, though.

Edwards Syndrome

Edwards syndrome, or Trisomy 18, is the second most common chromosomal mutation. It is estimated to occur in 1 out of 3,000 live births. Edwards syndrome is usually fatal, with less than 10% of babies born with this syndrome living more than one year. The syndrome causes heart defects, kidney abnormalities, and severe developmental delays.

Patau Syndrome

Patau syndrome is a result of an extra chromosome in the 13th location, and is also known as Trisomy 13. Like Edwards syndrome, less than 10% of babies born with Patau syndrome live more than one year, and many die shortly after birth if they are born alive at all. This syndrome occurs in an estimated 1 out of 10,000 live births. It can cause structural problems with the brain, severe mental retardation, and heart defects.

Less Common Conditions

Trisomies can and do occur on any chromosome other then 21, 18, or 13, but they rarely result in live births. There are isolated cases of children with rarer trisomies surviving for several years. A mutation at the 16th chromosome, for instance, is thought to occur in about 1% percent of all pregnancies, but these pregnancies usually result in miscarriages during the first trimester due to severe defects in the fetus.

Full trisomies occur when every cell in the body has the extra chromosome. In mosaic trisomies, only some cells contain the extra chromosome. People with partial trisomies have just part of the extra chromosome in their cells. Both mosaic and partial trisomies may be hereditary, while full trisomies are not. Symptoms and survival rates vary depending on the extent of the mutation.

Causes and Detection

Most of the time, modern ultrasound technology and prenatal screening can detect likely cases of chromosome mutation well before delivery. Still, some mutations are not discovered until birth. In general, the risk of delivering a baby with a trisomy mutation increases exponentially with the age of the mother at the time of birth. Most medical experts advise that the risk grows every year after the age of 35. Certain vitamin and mineral shortages may also be linked to the development of problems, but the research backing this is not conclusive. Most of the time, these sorts of mutations are simply a matter of chance.


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