Table of ContentsView AllTable of ContentsOverviewDown SyndromeEdwards SyndromesPatau SyndromeWarkany SyndromeTrisomy 16Trisomy 22Trisomy 9Kleinfelter SyndromeTriple X SyndromeXYY Syndrome
Table of ContentsView All
View All
Table of Contents
Overview
Down Syndrome
Edwards Syndromes
Patau Syndrome
Warkany Syndrome
Trisomy 16
Trisomy 22
Trisomy 9
Kleinfelter Syndrome
Triple X Syndrome
XYY Syndrome
Atrisomyis a genetic disorder in which a person has threechromosomesinstead of the usual two. The most well-known trisomy isDown syndrome, but there are others like Edwards syndrome, Patau syndrome, and Klinefelter syndrome that have distinct symptoms and characteristics.
Some trisomies cause few, if any, symptoms. Others can lead to severe birth defects that make life and pregnancy unsustainable.Prenatal screeningcan detect some of these genetic disorders before a child is even born.
This article explains how trisomies occur and why some are more severe than others. it also describes 10 different trisomies and the medical issues they can cause.
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What Causes a Trisomy?
To understand trisomies, it helps to first understand what genes and chromosomes are and their roles in human development.
Chromosomesare thread-like structures housed in the center (nucleus) of a cell that warehouse all the genetic information for each and every individual. Chromosomes are made up ofgenes, each of which carries coded information in the form ofDNA.
Each human cell contains a total of 46 chromosomes, 23 of which we inherit from our mothers and 23 of which we inherit from our fathers.
Of these, 22 pairs are calledautosomeswhich determine our unique biological and physical features. The 23rd pair aresex chromosomes—known as an X and a Y chromosome—which determine our biological sex.
When Problems Occur
In rare instances, a coding error may occur as a cell divides duringembryogenesis(the eight-week process where a fertilized egg develops into afetus). Instead of splitting into two identical chromosomes during the division, the chromosome will divide abnormally and end up with extra genetic material.
This can lead to either afull trisomy(in which a complete third chromosome is created) or apartial trisomy(in which only part of the chromosome is copied). From this point forward, the error will be repeated and repeated until embryogenesis is complete.
Translocation and MosaicismIn addition to full and partial trisomy, thereis also:Translocation trisomy: This is when a piece of a chromosome is moved and attached (translocated) to another complete chromosome.Mosaicism trisomy: This is when there are an abnormal number of chromosomes in a cell.
Translocation and Mosaicism
In addition to full and partial trisomy, thereis also:Translocation trisomy: This is when a piece of a chromosome is moved and attached (translocated) to another complete chromosome.Mosaicism trisomy: This is when there are an abnormal number of chromosomes in a cell.
In addition to full and partial trisomy, thereis also:
How Genetic Disorders Are Inherited
Consequences of a Trisomy
Different trisomies have different features. Autosomal trisomies often cause severe physical and intellectual disabilities (especially full trisomies that can sometimes lead to early death).
By contrast, trisomies involving sex chromosomes—for which females typically have two X chromosomes (XX) and males typically have an X and Y chromosome (XY)—tend to be far less severe.
Here are 10 different trisomies involving both autosomal and sex chromosomes:
Down Syndrome (Trisomy 21)
Down syndrome(trisomy 21) occurs when there is a full or partial extra copy of chromosome 21. It can also be the result of translocation or mosaicism;
There can also be significant intellectual disabilities and medical problems like hearing loss and heart defects,
Approximately 6,000 babies in the United States are born with Down syndrome each year. With appropriate care, most can have a relatively normal life span.
Edwards Syndrome (Trisomy 18)
Compared to Down syndrome, Edwards syndrome is relatively uncommon, affecting one of every 5,000 births.
Patau Syndrome (Trisomy 13)
Patau syndrome is the third most common autosomal disorder among newborns after Down syndrome and Edwards syndrome.
Warkany Syndrome (Trisomy 8)
Warkany syndrome(trisomy 8) is a common cause of miscarriage and, for newborns who survive, death usually occurs within the first months. Wakany syndrome is most commonly the result of mosaicism.
Babies born with Warkany syndrome typically have a cleft palate, distinctive facial features, heart defects, malformed joints, abnormal or missing kneecaps, and an abnormally curved spine (scoliosis).
Babies born with mosaic trisomy 8 can survive, but this is rare. Worldwide, mosaic trisomy 8 occurs in one out of every 25,000 to 50,000 live births.
Trisomy 16 is most often due to a complete or partial extra copy of chromosome 16. It may also be the result of mosaicism.
Trisomy 16 is the most common autosomal trisomy seen in miscarriages, accounting for at least 15% of first-trimester pregnancy losses.
By contrast, children with partial trisomy 16 can survive but often have significant intellectual disabilities as well as distinctive facial features, undersized lungs, and anatrial septal defect(a hole between the upper chambers of the heart).
Most fetuses with full trisomy 22 are miscarried before the first trimester. The physical and organ defects are so severe that babies who are born are unable to survive for more than a few hours or days. Trisomy 22is the second most common chromosomal cause of miscarriages.
On rare occasions, babies with mosaic trisomy 22 do survive but often have serious medical issues like heart defects, kidney problems, intellectual disability, and developmental delays.
Trisomy 9 is a rare disorder in which a full trisomy is usually fatal within the first 21 days of life. Newborns with trisomy 9 will have a smaller head, distinctive facial features (including a bulbous nose and sloping forehead), a deformed heart, kidney problems, and severe muscle and skeletal defects.
Babies born with partial or mosaic trisomy 9 have a far greater chance of survival. This is especially true with mosaic trisomy 9 in which physical defects and intellectual disabilities don’t necessarily impede a child’s development.
Since mosaic trisomy 9 was first identified in 1973, only a handful of cases have been cited in the medical literature.
Klinefelter Syndrome (XXY Syndrome)
Klinefelter syndrome, also known as XXY syndrome, is a condition affecting males in which there is an extra X chromosome. People with Klinefelter syndrome typically produce littletestosteroneand, as a result, have smaller muscle mass and less facial and body hair.
Other possible symptoms include smalltesticles, enlarged breasts (gynecomastia), and infertility. Some people with Klinefelter syndrome may have learning disabilities, typically language-oriented, although intelligence is usually not affected.
Klinefelter syndrome is one of the more common trisomies, affecting one of every 500 or 1,000 babies. Even so, the severity of symptoms can vary from one person to the next, with some having no notable signs or symptoms.
Triple X Syndrome (Trisomy X)
Some females are born with triple X syndrome (trisomy X) in which there is an extra X chromosome. The trisomy is not associated with physical features and often causes no medical symptoms.
A small proportion of those with triple X syndrome will havemenstrual problemsor learning disabilities, but the vast majority develop normally and without impairment of any sort.
Triple X syndrome is reported in one of every 1,000 live births. Because it causes no overt symptoms, the number of actual cases is likely greater.
Most males born with XYY syndrome, in which there is an extra Y chromosome, have no medical concerns of any sort. If anything, they can sometimes be taller than average.
On the flip side, people with XYY syndrome may be at an increased risk of learning disabilities and developmental delays. Even so, the impairment tends to be mild.
Most adults with XYY syndrome have normal sexual development and are able to conceive children.
XYY syndrome is thought to affect one of every 1,000 live births. Because there are few symptoms, it is thought that only one in eight cases is officially diagnosed.
18 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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