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What is Down Syndrome?

History, Causes and Characteristics


Updated June 04, 2014

What is Down Syndrome?

A boy with Down syndrome


What is Down Syndrome?

Down syndrome is a congenital condition caused by an extra chromosome. The presence of an extra number 21 chromosome causes the distinctive facial features, physical characteristics and the cognitive impairments seen in people with Down syndrome. While people with Down syndrome have some characteristics in common, it is very important to remember that each person with Down syndrome is an individual with strengths and weaknesses. Never make assumptions about a person’s abilities based on their diagnosis.

To really understand what it means to have an extra chromsome, it helps to have some basic information about chromosomes.


Chromosomes are basically the packages of genetic information that are found in every cell of the human body. Most humans have 46 chromosomes arranged in pairs for a total of 23 pairs of chromosomes. There are twenty-two pairs of what are called autosomes and one pair of sex chromosomes. Women have two X chromosomes and men have an X and a Y chromosome. People with Down syndrome have an extra number 21 chromosome - a condition that is also called trisomy 21. Instead of 46 chromosomes, they have 47. Having three copies of the genetic material on chromosome 21 is what causes Down syndrome.


Down syndrome was first described by Dr. John Langdon Down in 1866. He was a physician in England with a special interest in mental retardaton. While he was the first to describe some of the distinctive characteristics of people with Down syndrome, it wasn't until 1959 that the cause of Down syndrome, an extra chromosome 21, was discovered by Dr. Jerome Lejeune who was studying chromosomes (a speciality called cytogenetics). Chromosomes can be seen under a microscope and Dr. Lejeune was the first to see 47 chromosomes in the cells of an individual with Down syndrome rather than 46.

The Purpose of Lists

It is somewhat unfair and misleading to simply list all of the symptoms of Down syndrome because each person with Down syndrome is unique, and no one person with Down syndrome will have all of the symptoms listed. The purpose of this list is to provide a basic overview and give you the knowledge that you need to be proactive in caring for your child.

Facial and Physical Features

Individuals with Down syndrome have some distinct facial features that cause them to resemble one another, as well as their families. They can have almond shaped eyes with epicanthal folds, light-colored spots in their eyes called Brushfield spots, a small somewhat flatter nose, a small mouth with a protruding tongue, and small ears. They also have round faces and somewhat flatter facial profiles.

Other physical features seen in people with Down syndrome include a single crease across the palms of their hands, short stubby fingers and a fifth finger that curves inward called clinodactyly. They have a smaller head that is somewhat flattened in the back (brachycephaly), and straight hair that is fine and thin. In general, they tend to have short stature with short limbs, and can have a larger than normal space between the big and second toes.

Medical problems

Children with Down syndrome are at higher risk to develop a number of specific medical problems. While most people with Down syndrome do not have serious medical problems, it is good to be aware of potential complications so that appropriate medical treatment can be sought early before serious complications arise.

Almost all infants with Down syndrome have low muscle tone which is called hypotonia. This means that their muscles are somewhat weak and they appear floppy. While this isn’t a medical problem per se, it is important because muscle tone can affect a child with Down syndrome’s ability to learn and grow. Hypotonia cannot be cured but it generally improves over time.

Hypotonia can also lead to some of the orthopedic or bone problems such at atlantoaxial instability that some people with Down syndrome can have.

The majority of children with Down syndrome will have some type of vision problem such as nearsightedness, farsightedness, crossed-eyes and even blocked tear ducts. About 40% of babies with Down syndrome are born with heart defects which can range from mild to severe. Somewhere between 40-60% of babies with Down syndrome will have some form of hearing loss. Other problems seen less frequently include gastrointestinal defects, thyroid problems and very rarely leukemia.

Mental Retardation

All individuals with Down syndrome have some degree of mental retardation. They learn more slowly and have difficulties with complex reasoning and judgement, but they do have the capacity to learn. It is impossible to predict the degree of mental retardation in an infant with Down syndrome at birth (just as it is impossible to predict the IQ of any infant at birth).

It is very important that infant and people with Down syndrome receive the support, guidance, education and appropriate treatments needed to maximize their potential and to allow them to live fulfilling lives.


Stray-Gunderson, K., Babies with Down Syndrome - A New Parents Guide, Woodbine House, 1995.

Chen, H., Down syndrome, Emedicine, 2007

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