What Is Down Syndrome?

Down syndrome is a lifelong genetic condition that begins to have effects before birth and can significantly impact many aspects of a person’s life.

People who have Down syndrome can experience physical effects as well as cognitive challenges. However, the severity of the condition varies from person to person, and many people living with Down syndrome can lead happy, productive, and healthy lives.

This article describes the types of Down syndrome, its causes, risk factors, symptoms, screening tests, and long-term outlook.

Types of Down Syndrome

Down syndrome is congenital, which means that it is present at birth. In fact, Down syndrome is present at conception. The condition occurs when a fertilized egg has an extra copy of chromosome 21. Down syndrome is also called trisomy 21.

The types of Down syndrome differ based on chromosomal patterns, as follows:1

  • Trisomy 21: This is the most common type of Down syndrome. It occurs when a person has three copies of chromosome 21 in each cell of their body. A person’s typical number of chromosomes is 46 (in 23 pairs). A person with Down syndrome has 47 chromosomes if all other chromosome pairs are typical.
  • Mosaic Down syndrome: This type of Down syndrome occurs when there is a mixture of some cells in the body with trisomy 21 and some cells in the body without an extra chromosome 21. The symptoms can be similar to symptoms of full trisomy 21, but sometimes the effects are milder. It is seen in about 2% of people diagnosed with Down syndrome.
  • Translocation: This type occurs when a person has extra chromosome 21 genetic material attached to another chromosome. With this type of Down syndrome, the person may have 46 chromosomes in their cells. About 3% of people diagnosed with Down syndrome have this type.

Causes and Risk Factors

The extra genetic material from the third copy of chromosome 21 causes the body to develop differently. This occurs whether a person has full trisomy 21, mosaic Down syndrome, or translocation.

This produces changes in the developing fetus’s physical features. Many of the changes are present at birth, and some can develop as the child grows into adolescence and adulthood.

Risk factors for Down syndrome include:2

  • Advanced age of the birthing parent, especially age 35 and older at the time of conception
  • A family history of Down syndrome or another chromosomal disorder in a parent or sibling


Down Syndrome and Genetics

The genetic pattern of Down syndrome occurs due to the presence of an extra copy of chromosome 21 in the parents’ egg cell or sperm cell. A child should normally receive only one copy of each chromosome from each parent—resulting in two copies of each chromosome in every one of the child’s cells.

When cells from either parent have two copies of any chromosome, this results in trisomy (the presence of a third copy) in all of the growing baby’s cells throughout their life. Trisomy cannot be repaired with any type of medical intervention.

The process that causes an egg cell or a sperm cell to have an extra copy of a chromosome is called nondisjunction.3

 It occurs during the formation of the egg and the sperm cell, prior to the embryo’s conception.

Down syndrome is not usually inherited—most people who have Down syndrome are the first in the family to have it, and their parents do not have the condition. However, it can be passed from parent to child if a person who has Down syndrome becomes pregnant or impregnates someone.

Read more: Very Well Health

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