Trisomy- Definition, Types, Characteristics, Diagnosis

Trisomy is a genetic condition or chromosomal disorder that results in an extra copy of chromosomes.

  • There are 23 pairs of chromosomes present in humans.
  • An individual with trisomy conditions has 47 chromosomes instead of 46 chromosomes.
  • Normally, humans have two copies of chromosomes, but they have three copies of chromosomes during this condition.
  • For example, down syndrome (trisomy 21) is caused by the presence of three copies of chromosome 21 instead of the usual two pairs.
  • Mostly trisomy conditions appear after the age of thirty-five and increase the risk of having a baby at this age.
  • As the trisomy condition appears spontaneously, it is difficult to prevent it.
  • Due to the alteration in the number or genetic structure of the chromosome, a chromosomal condition is caused.
  • Trisomy-affected children have delayed development and intellectual abilities.

Types of trisomy disorders

There are different types of trisomy disorders, and these trisomy disorders are identified on the basis of chromosomal defect, which means identifying the number of copies of chromosomes. Different trisomy conditions are:

  1. Trisomy 13 (Patau syndrome)
  2. Trisomy 18 (Edward syndrome)
  3. Trisomy 21 (Down syndrome)

Trisomy 13 (Patau syndrome)

One in 3,000 pregnancies is affected by the patau syndrome alone in Victoria. Patau syndrome generally has three sets of chromosome 13 instead of two and is also known as trisomy 13.

Trisomy 13 (Patau syndrome)
Trisomy 13 (Patau syndrome)

Characteristics of Trisomy 13 (Patau syndrome) are as follows:

  • An individual has a small skull, also known as microcephaly.
  • The skull has an abnormal opening.
  • The part of the brain malfunctions.
  • The eyes have certain kinds of structural defects.
  • May have additional fingers or toes, and this condition is called polydactyly.
  • Congenital heart disorders such as defects in the ventricular septal.
  • Defect in the neural tube (is a gap through where the spinal cord, meninges, and blood vessels protrude out in the vertebrae. This condition is also called myelomeningocele.
  • Sex organs also malfunction. 

Trisomy 18 (Edwards syndrome)

Edwards syndrome is the condition where the person has three copies of chromosome 18 instead of two. Edwards syndrome is also called trisomy 18.

Trisomy 18 (Edwards syndrome)
Trisomy 18 (Edwards syndrome)

Some of the characteristics of Trisomy 18 (Edwards syndrome) are:

  • Different physical organs like kidneys, ureters, heart, lungs, and diaphragm show irregularity or do not function properly.
  • The person has a small skull, and this condition is called microcephaly.
  • Hands and feet malfunction.
  • Missing of the thumb and club feet.
  • The web is present between the fingers and toes, and this condition is called syndactyly.
  • Defect in the neural tube (is a gap through where the spinal cord, meninges, and blood vessels protrude out in the vertebrae. This condition is also called myelomeningocele.
  • Sex organ malfunction.

Trisomy 21 (Down syndrome)

Down syndrome is the condition where the person has three copies of chromosome 21 instead of two. Down syndrome is also called trisomy 21. There are three types of the down syndrome; the most common is trisomy 21, where either the father’s sperm or the mother’s egg cell contains extra pairs of chromosomes. 

In Mosaic syndrome, an extra pair of chromosomes appear spontaneously as the embryo develops. Similarly, there is another down syndrome called translocation down syndrome.

Trisomy 21 (Down syndrome)
Trisomy 21 (Down syndrome)

Characteristics of Trisomy 21 (Down syndrome) include:

  • Slightly upward slanting of the eyes: All people with Down syndrome have their eyes slanted slightly upward. Sometimes there can be a small fold of the skin on the inside of the eye; this condition is known as ‘epicanthic fold’. Similarly, there may be small white patches on the edge of the iris of the eye, and this condition is known as ‘Brushfield spot’.
  • Characteristics of facial shape: the person with Down syndrome tends to have a rounded and flat face.
  • Smaller stature: as compared to normal babies, babies with this syndrome have less weight and are usually smaller at the time of birth. Children having this syndrome tends to grow slower than that normal children, and the condition is similar to that of adults.
  • All people with down syndrome have certain kinds of developmental and learning disabilities.

Some of the signs of trisomy conditions during the pregnancy include:

  • The baby is surrounded by a huge amount of amniotic fluid, and this condition is known as polyhydramnios.
  • There is the presence of only one umbilical cord artery.
  • The placenta is smaller than expected.
  • The size of the baby is still small during the gestational date.
  • The babies seem to be less active than expected.
  • It may include congenital defects like heart irregularities and cleft palate.

Instruments used for the diagnosis of the trisomy conditions

Some of the parental tests that can detect trisomy conditions include:

  • Ultrasound scans: during this, sound waves are used to create the images, and after this, the pictures are observed to see any abnormalities.
  • Maternal serum screening: It is a kind of specialized blood test to observe trisomy conditions.
  • Amniocentesis: here, the sample of amniotic fluid is taken and examined for any defects.
  • Chorionic villus sampling: chorion is the tissue that will ultimately become the placenta. Thus, a sample is extracted from the cells of the chorion and examined to find any irregularities. 
  • Non-invasive parental screening (NIPT): It is a screening test that measures the amount if fetal DNA circulation in the mother’s blood.

How are trisomy conditions treated?

As the trisomy condition is a lifelong condition, it requires lifelong treatment to prevent any other symptoms from appearing. Some of the treatment conditions include:

  • Performing surgery to remove any physical abnormalities.
  • Providing educational support and knowledge related to these syndromes.
  • Physical and behavioral therapy.
  • Medical treatment with the help of medicines to treat the related symptoms.


  6. Merck Manual. Down Syndrome (Trisomy 21). ( Accessed 5/3/2022.
  7. Centers for Disease Control and Prevention. Down Syndrome. ( Accessed 5/3/2022.

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