Types of Genetic Trisomy Disorders

A trisomy is a genetic disorder in which a person has three chromosomes instead of the usual two. The most well-known trisomy is Down 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 screening can 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.

What Causes a Trisomy?

To understand trisomies, it helps to first understand what genes and chromosomes are and their roles in human development.

Chromosomes are 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 of genes, each of which carries coded information in the form of DNA.

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 called autosomes which determine our unique biological and physical features. The 23rd pair are sex 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 during embryogenesis (the eight-week process where a fertilized egg develops into a fetus). 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 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 until embryogenesis is complete.

Trisomies are typically classified by the specific chromosome that has been affected. For instance, 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.

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.

In addition to causing birth defects, trisomies can cause miscarriage. In fact, more than half of all miscarriages are thought to be caused by a chromosomal defect.

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;

Down syndrome is characterized by distinctive physical features, including:

• A flattened face and bridge of the nose
• Slanted, almond-shaped eyes
• A short neck
• Small ears
• A tongue that tends to stick out
• Small hands and feet
• A single line across the palm of the hand (palmar crease)
• Small, curved pinky fingers
• Poor muscle tone
• Shorter stature

There can also be significant intellectual disabilities and medical problems like hearing loss and heart defects,

Edwards Syndrome (Trisomy 18)

Edwards syndrome (trisomy 18) is usually caused by an extra chromosome 18. Only around 5% of cases are due to translocation.

Edwards syndrome is characterized by low birth weight, an abnormally small head, and heart, kidney, and lung defects. While a few children with Edwards syndrome survive to adolescence, the majority die within the first year (and often the first days) of life.

Compared to Down syndrome, Edwards syndrome is relatively uncommon, affecting one of every 5,000 births.

Patau Syndrome (Trisomy 13)

Most cases of Patau syndrome (trisomy 13) are related to a full trisomy. Only a few are caused by translocation or mosaicism.

Children with Patau syndrome will often have cleft lips and palates, extra fingers or toes, heart defects, severe brain abnormalities, and malformed internal organs. Symptoms are generally so severe that a baby with Patau syndrome rarely lives past the first month.

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

Trisomy 16 is most often due to a complete or partial extra copy of chromosome 16. It may also be the result of mosaicism.

Full trisomy 16 is incompatible with life. While most fetuses are spontaneously aborted by the 12th week of pregnancy, a few have survived into the second trimester.

By contrast, children with partial trisomy 16 can survive but often have significant intellectual disabilities as well as distinctive facial features, undersized lungs, and an atrial septal defect (a hole between the upper chambers of the heart).

Children with mosaic trisomy 16 are least affected by the disorder and may grow to a normal weight and height by the time they are toddlers. Some may be otherwise healthy, but others may have congenital problems such as heart defects and developmental delays.

Trisomy 22

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 22 is 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

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.

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 little testosterone and, as a result, have smaller muscle mass and less facial and body hair.

Other possible symptoms include small testicles, 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 have menstrual problems or 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.

XYY Syndrome

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.