Trisomy is when three copies of a chromosome are present instead of two (all chromosomes normally come in pairs). While most parents-to-be are familiar with Down syndrome and will undergo prenatal screening to detect it, there are other, potentially more serious trisomies that may occur, including Edwards syndrome, Patau syndrome, and others.
Some may cause few, if any, symptoms. Others can lead to severe defects that make life—or even the pregnancy—unsustainable.
What Are Trisomies?
Genes that contain all of the DNA-coded information related to physiologic makeup and metabolic function are found on chromosomes. Each human cell nucleus typically contains 46 chromosomes, 23 of which we inherit from each genetic parent.
Of these, 22 pairs are autosomes, which determine our unique biological and physiological features. The 23rd pair is sex chromosomes (X or Y), which largely determine biological sex.
In rare instances, a coding error may occur when a cell divides during fetal development. Instead of splitting cleanly into the two identical chromosomes, the newly divided chromosome will have extra genetic material.
This can lead to either a full trisomy (in which a complete third chromosome is created) or a partial trisomy (in which only part of the chromosome is copied). From this point forward, the error will be repeated and repeated as the cell continues to divide.
Down syndrome, the most common genetic disorder in humans, is referred to as trisomy 21 because there is an extra copy of chromosome 21 in the nucleus of each cell. Other genetic disorders are similarly named.
Causes and Consequences
Trisomies affecting the sex chromosomes—in which genetic females typically have two X chromosomes (XX) and genetic males have an X and Y chromosome (XY)—tend to be less severe. Autosomal trisomies often cause serious physical and intellectual disabilities, particularly full autosomal trisomies, for which early death is common.
In addition to birth defects, trisomies can undermine the viability of a pregnancy. In fact, it is believed that more than half of all miscarriages are directly associated with a chromosomal defect. Of these, many are due to trisomies.
No one knows for sure why chromosome 21 is so vulnerable to trisomy. Of all the trisomies identified by researchers, Down syndrome is known to affect nearly one of every 800 births worldwide. These other trisomies are far less common but are worth knowing about.
Types of Trisomy
There are a number of different types of trisomy. Some can result in a live baby, while others may cause miscarriage in the early months of pregnancy.
Edwards Syndrome (Trisomy 18)
Edwards syndrome (trisomy 18) is usually caused by an extra chromosome 18. About 5% of cases are due to an error known as translocation in which the building blocks of one chromosome is inserted into another.
Edwards syndrome is rare, affecting only one of every 5,000 births.
Edwards syndrome is characterized by low birth weight, an abnormally small head, and defects in the heart, kidneys, lungs, and other organs. While a few children with Edwards syndrome survive to adolescence, the majority die within the first year (and often the first days) of life.
Patau Syndrome (Trisomy 13)
Most cases of Patau syndrome (trisomy 13) are related to a full trisomy; a very small proportion is caused by translocation or a similar condition known as mosaicism in which the chromosomal building blocks are rearranged.
Patau syndrome is the third most common autosomal disorder among newborns after Down syndrome and Edwards syndrome.
Children with Patau syndrome will often have cleft lips and palates, extra fingers or toes, heart defects, severe brain abnormalities, and malformed or rotated internal organs. The severity of symptoms is such that a baby with Patau syndrome rarely lives past the first month.
Warkany Syndrome (Trisomy 8)
Warkany syndrome (trisomy 8) is a common cause of miscarriage and usually results in newborn death within the first months. Babies born with Warkany syndrome typically have a cleft palate, distinctive facial features, heart defects, joint malformation, abnormal or missing kneecaps, and an abnormally curved spine (scoliosis).
Babies born with mosaic trisomy 8 (T8MS) can survive, but the condition is rare. Worldwide, T8MS occurs in approximately one out of every 25,000 to 50,000 live births.
Trisomy 16
Full trisomy 16 is incompatible with life. While most fetuses with this abnormality are spontaneously aborted by the 12th week of gestation, a few have survived into the second trimester.
Trisomy 16 is the most common autosomal trisomy seen in miscarriages, accounting for at least 15% of first-trimester pregnancy losses.
By contrast, the chances of survival of children with mosaic trisomy 16 used to be considered bleak with most deaths occurring in early infancy.
Advances in genetic research have since shown that some children previously unidentified with mosaic trisomy 16 have no abnormalities of any sort and that the risk of miscarriage and birth defects is directly related to the number of cells carrying the chromosomal mutation.
With that being said, more than half of babies with mosaic trisomy 16 will have fetal abnormalities. Symptoms of trisomy 16 may include musculoskeletal defects, distinctive facial features, undersized lungs, and an atrial septal defect (a hole between the upper chambers of the heart). Life expectancy for children with mosaic trisomy 16 depends on the extent of these types of problems.
Males will often have hypospadias in which the opening of the urethra develops on the shaft of the penis rather than at the end. Development delays may occur but are less common than other trisomies.
Trisomy 22
Most fetuses with full trisomy 22 are miscarried before the first trimester. The severity of physical and organ defects is such that babies carried to term are unable to survive for more a few hours or days.
Trisomy 22is the second most common chromosomal cause of miscarriages.
Some babies with mosaic trisomy 22 do survive. The severity of birth defects is determined by the number of cells with the mutated chromosomal copy. Characteristic detects include heart abnormalities, kidney problems, intellectual disability, muscle weakness, and cognitive and developmental delays.
Trisomy 9
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 often severe muscle and skeletal malformations.
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 organ defects tend to be less severe and intellectual disabilities don't necessarily impede basic language, communication, or social-emotional development.
Since the disorder was first identified in 1973, few cases of mosaic trisomy 9 have been positively identified in medical literature.
Klinefelter Syndrome (XXY Syndrome)
Klinefelter syndrome, also known as XXY syndrome, is a condition affecting males caused by an additional X chromosome. Individuals with Klinefelter syndrome typically produce little testosterone, resulting in reduced muscle mass, facial hair, and body hair.
Characteristic symptoms include small testicles, delayed development, breast enlargement (gynecomastia), and reduced fertility. The severity of symptoms can vary dramatically.
Klinefelter syndrome is common, affecting .1% to .2% of newborn males.
Some people with Klinefelter syndrome may also have learning disabilities, which are typically language-oriented, although intelligence will usually be normal. Testosterone replacement therapy is often used to treat the disorder alongside assisted fertility treatments for those wanting to father children.
Triple X Syndrome (Trisomy X)
Some females are born with triple X syndrome, involving an extra X chromosome. Triple X syndrome, also known as XXX syndrome, is not associated with physical features and often causes no medical symptoms at all.
XXX syndrome is reported in about 1 in 1,000 newborn females. Because XXX syndrome may not cause obvious symptoms, however, it is thought to be underdiagnosed.
A small proportion of those affected may have menstrual irregularities as well as learning disabilities, delayed speech, and compromised language skills. When trisomy x causes symptoms, treatment usually involves physical and speech therapy and classroom support. However, most people with trisomy x will develop normally and without impediment.
XYY Syndrome
Most males born with an extra Y chromosome have no distinctive physical features or medical issues. If anything, individuals with XYY syndrome can sometimes be taller than average and may have an increased risk of learning disabilities, as well as delayed speech and language skills.
XYY syndrome is reported to affect 1 in 1,000 newborn males. Because there are few symptoms, however, it's thought that a diagnosis is made in only about 15% of cases.
The impairment, if any, tends to be mild. Most adults with XYY syndrome have normal sexual development and are able to conceive children.
Frequently Asked Questions
What are the 3 most common trisomy anomalies?
The three most common trisomies are Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). Children with Down syndrome often have a good prognosis, with a life expectancy of up to 60 years. Most children with Edwards syndrome and Patau syndrome, however, die in infancy.
What is the life expectancy of someone with trisomy X?
Trisomy X does not affect life expectancy. Most people born with trisomy X lead normal lives, though there is an increased risk of learning disabilities.
How common is trisomy 16 miscarriage?
Some sources estimate that trisomy 16 causes up to 15% of all miscarriages that occur in the first trimester.