Navigation Symbol

What Exactly is Down Syndrome?

By: Carine Human

Every cell in the human body has a nucleus and this is where genetic material is stored in genes. The codes responsible for all our inherited traits, are called genes and they are grouped along rod-like structures called chromosomes. Characteristically, the nucleus of each cell contains 23 pairs of chromosomes. Half are inherited from each parent. Down Syndrome occurs when an individual has a full or partial additional copy of chromosome 21.

This extra genetic material alters development and causes the characteristics associated with Down Syndrome. Common physical traits of Down Syndrome include low muscle tone, small build, an upward slant to the eyes, and a single deep crease across the center of the palm - however, each person with Down Syndrome is a unique individual and could have these physiognomies to different degrees, or not at all.

Typical traits associated with Down Syndrome


The occurrence of Down Syndrome

  • One in 1 000 babies born in developed countries and 1 in 650 babies born in developing countries is affected by down Syndrome.
  • 80% of children affected by Down Syndrome are born from mothers under the age of 35, though women over the age of 35 are at a higher risk of having a child with Down Syndrome.

The discovery of Down Syndrome

French physician Jerome Lejeune identified Down Syndrome in 1959 as a chromosomal condition. He observed 47 instead of the usual 46 chromosomes present in each cells of those with Down Syndrome. Later, it was determined that an extra partial or whole copy of chromosome 21 results in the characteristics associated with Down Syndrome. In 2000, an international team of scientists successfully identified and catalogued each of the approximately 329 genes on chromosome 21, which opened the door to great advances in Down Syndrome research.

The three Different Types of Down Syndrome


"Nondisjunction" refers to an error in cell division. This "error" usually causes Down Syndrome - an embryo ending up with three copies of chromosome 21 instead of the usual two. Before, or at conception, a pair of 21st chromosomes in either the sperm or the egg does not separate. During development of the embryo, the extra chromosome is replicated in every cell of the body. About 95% of cases is due to this and is called trisomy 21.

Typical cell division


Trisomy 21 (Nondisjunction) cell division



In Mosaic Down Syndrome or Mosaicism, there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. The cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism is the least common form of Down Syndrome. Approximately 1% of all cases of Down Syndrome. Individuals with Mosaic Down Syndrome may have fewer characteristics typically of Down Syndrome.

Mosaic cell division



With translocation, the total number of chromosomes in the cells remains 46; though, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The existence of the extra full or partial chromosome 21 causes the features of Down Syndrome. Only about 4% of Down Syndrome cases are due to Translocation.

The causes of Down Syndrome

It is still unknown what the case is of the extra full or partial chromosome. Maternal age is the only factor that has been linked to an increased risk of Nondisjunction or Mosaicism.

The extra 21st chromosome resulting in Down Syndrome can originate from either parents. About 5% of the cases have been traced to the father.

The likelihood of having a child with Down Syndrome

Down Syndrome affects people of all races and economic levels. Older women, however, have an increased chance of having a child with Down Syndrome. A 35 year old woman has about a one in 350 chance, and this chance increases gradually to 1 in 100 by age 40. At age 45 the frequency becomes approximately 1 in 30. The age of the mother does not appear to be linked to the risk of translocation.

The likelihood of having a child with Down Syndrome

Genetic counseling for parents is increasing in importance as many couples are having children later in life. Yet, it is perceived that few physicians are fully informed about advising patients about the incidences of Down Syndrome, advancements in diagnosis, and the protocols for care and treatment of babies born with Down Syndrome.

Down Syndrome in families

Down Syndrome is a genetic condition (relating to the genes), but only 1% of all cases of Down Syndrome have a hereditary component (passed from parent to child through the genes). In trisomy 21 (nondisjunction) and mosaicism, heredity is not relevant. However, in one third of cases of Down Syndrome resulting from translocation, there is a hereditary component - which is about 1% of all Down Syndrome cases.

The mother's age does not seem to be linked to the risk of translocation, as most cases are random events. Though, in one third of cases, one parent is a carrier of a translocated chromosome.

Once a woman had a baby with trisomy 21 (nondisjunction) or translocation, it is projected that her chances of having another baby with trisomy 21 is 1 in 100 up until age 40.

In translocation, the risk of recurrence is about 3% if the father is the carrier and 10-15% if the mother is the carrier. Genetic counseling can help determine the origin of translocation.

Diagnosing Down Syndrome


There are two categories of tests for Down Syndrome that can be performed prenatally. The first is a screening tests and second, a diagnostic test. Prenatal screening estimates the chance of the fetus having Down Syndrome. These tests only provide a probability. Diagnostic tests, on the other hand, can provide a definitive diagnosis with nearly 100% accuracy.

Screening tests mostly involve a blood test, along with an ultrasound (sonogram). The blood tests (or serum screening tests) quantifies amounts of various substances in the blood of the mother. Together with her age, this is used to estimate her chance of having a child with Down Syndrome. These blood tests are often performed in combination with a detailed sonogram to check for "markers" (characteristics that some researchers consider may have a significant association with Down Syndrome). New progressive prenatal screens are now able to detect chromosomal material from the fetus that is circulating in the mother's blood. They are non-invasive tests (unlike the diagnostic tests below), but they provide a high accuracy rate. Screening will not definitively diagnose Down Syndrome, however. Both prenatal screening and diagnostic tests are now routinely offered to women of all ages. Diagnostic procedures available for prenatal diagnosis are chorionic villus sampling (CVS) and amniocentesis. These procedures carry an up to 1% risk of causing a spontaneous termination (miscarriage), are nearly 100% accurate in diagnosing Down Syndrome. Amniocentesis is usually performed between 15 and 20 weeks of gestation (2nd semester) and CVS in the between 9 and 14 weeks (1st trimester).


Down Syndrome is often identified at birth by the observation of certain physical trait. These include low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile and an upward slant to the eyes. These features may also be present in babies without Down Syndrome, therefor a chromosomal analysis called a karyotype is done to confirm the diagnosis. Doctors draw a blood sample to examine the baby's cells to confirm a karyotype. The chromosomes are photographed and then group them by size, number, and shape. By examining the karyotype, doctors can then effectively diagnose Down Syndrome. Also, another genetic test called FISH can apply similar principles and confirm a diagnosis in a shorter amount of time.

Karyotype Of A Female With Trisomy 21


The Impact of Down Syndrome on Society?

Persons with Down Syndrome possess varying degrees of cognitive delays, from very mild to severe, although most people with Down Syndrome have cognitive delays that are mild to moderate. Individuals with Down Syndrome are also becoming increasingly integrated into society and community organizations, i.e schools, health care systems, work forces, and social and recreational centres. They are also living longer than ever before,due to advances in medical technology. In 1910, children with Down Syndrome were expected to live up to age nine, but then with the discovery of antibiotics, the average survival age increased to 19 or 20. Nowadays, with advancements in clinical treatment, most particularly corrective heart surgeries, as many as 80% of adults with Down Syndrome reach age 60, and many even older. Widespread public education and acceptance should be even more advocated for in order for acceptance, understanding and support of individuals with Down Sydrome and their carers and loved ones.