Down Syndrome: Definition, Causes And Different Types

Down syndrome is not an illness but a genetic disorder also known as trisomy 21

In the following article,  we will explain what Down syndrome is, what the main types and characteristics are, how to diagnose it, and what can be done to improve the quality of life  of adults and children living with this syndrome. 

What is Down syndrome?

Down syndrome is a genetic disorder characterized by the presence of a full or partial extra copy of chromosome 21.  Chromosomes are organic structures that contain DNA; in humans, each cell usually comprises 23 pairs of these chromosomes, and in total 46. 

The reason why Down syndrome is also known as trisomy 21 is that people that have this condition have three chromosomes instead of two in their 21 pair. This trisomy is the most common human genetic variation, and it happens without any apparent cause. 

The syndrome was named after British physician John Langdon Haydon Down, who in 1862 described the genetic condition we now know as Down syndrome. It wasn't until 1958, however, when French pediatrician and geneticist Jérôme Lejeune discovered the existence of a third chromosome in the 21 pair, that medicine started researching diseases and their connection to gene copy number. At present,  the estimated incidence of Down Syndrome is 1 in 1000 live births worldwide. 

Among the main characteristics of Down syndrome we can mention upward slanting eyes, short stature, and decreased or poor muscle tone. Similarly to all newborns, babies that display this condition will also display family features aside from the ones associated with this condition. 


Down syndrome is not an illness and therefore does not have a cure or treatment. It is called "syndrome" because it represents a group of symptoms which consistently occur together, that can be pathological or not, and that have in common a genetic cause: an extra chromosome. 

Comorbidity means that aside from the primary condition, in this case Down syndrome, other additional diseases will be present, although not all will have the same characteristics in all patients. 

Some of the diseases that can occur are  heart disease, stomach disease, thyroid dysfunctions, hematological variations which can cause leukemia, lung issues such as pneumonia or a collapsed lung, muscle and ophthalmological problems. 

Down syndrome and intellectual and developmental disabilities

People who have Down syndrome also struggle with intellectual and developmental disabilities (formerly known as mental retardation, the term was eliminated because it had become pejorative). This means difficulty when performing tasks within the accepted parameters specified by a standardized reference. 

The intellectual and developmental disabilities do not represent a disease either, but a condition that is determined by learning difficulties and social obstacles in our societies. 

One of the main characteristics of people with Down syndrome is extroversion and friendliness
One of the main characteristics of people with Down syndrome is extroversion and friendliness

Main types and causes

Although a specific cause has yet to be determined, scientists have associated the incidence of Down syndrome with maternal age (generally speaking, at 35-40 years old, women run a higher risk; however, this can also vary). Hereditary factors were indicated in only 1% of the cases recorded. 

Popular belief suggests that Down syndrome has "levels" or "stages" when, in fact,  there are three types of Down syndrome: trisomy 21, translocation, and mosaicism. 

The classification is not dependant on physical traits or degree of intellectual and developmental disabilities but on genetic information which indicates chromosomal variation. 

1. Free trisomy 21

The most common type of Down syndrome happens during the first stages of cellular reproduction.  The condition occurs when there are three, rather than two, number 21 chromosomes present in every cell of the body. Instead of the usual 46 chromosomes, a person with Down syndrome has 47. 

2. Chromosomal translocation

Translocation accounts for 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.

3. Mosaicism

This type of trisomy is the least frequent one. During cell division, some of the newly created cells have three copies of chromosome 21, and the other newly formed cell only has one chromosome.

This way some cells with three chromosomes are formed (trisomic cells) while the other cells follow their normal genetic development. It's because of this cellular mixture with different chromosomal traits that this type of trisomy is called mosaicism. 

Because not all cells have trisomic chromosomes in the 21 pair, patients who have this condition tend to display less pronounced Down syndrome physical traits and can also have  a lower degree of intellectual and developmental disabilities. This depends, however, on the number of trisomic cells created in the body. 

There are different types of Down syndromes with specific characteristics
There are different types of Down syndromes with specific characteristics

Diagnostic tests 

Nowadays, incidents of Down syndrome can be detected before birth using prenatal medical procedures such as chronic villus sampling (CVS) or amniocentesis (analysis of the amniotic fluid)

On the other hand, in newborns, it can be difficult to detect the physical characteristics of the syndrome, Down or another one for that matter, so after birth, doctors perform a chromosomal analysis called a karyotype. 

Early detection refers to a range of specialized programs and resources that professionals provide to very young children with Down syndrome and their families. One of the goals is to become aware of associated diseases and ways to prevent them. 

Quality of life and life expectancy 

The quality of life reflects the perceived well-being of a person, including physical health, family, education, employment, safety, wealth, and security which is why the concept has been associated with the opportunities that Down syndrome patients have in today's society. 

Many people have been fighting against the stigma associated with this condition, and now people who have Down syndrome are experiencing an improved quality of life. This can be observed in areas such as access to education, leisure activities and employment.  

Moreover, in today's society,  life expectancy for a Down syndrome patient is 60 years which is why one of the main areas for investigation is promoting an independent life, and help with transitioning into adulthood and old age by identifying the necessary coping strategies. 

World Down Syndrome Day is observed on the 21st March, which offers people the opportunity to learn about the causes and characteristics of this condition, as well as discover ways of fighting against the stigma associated with the people that have it.


National Down Syndrom Society,   

Schalock, R., Brown, R. & Brown, I. (2002). Conceptualization, measurement, and application of quality of life for persons with intellectual disabilities: report of an international panel of experts.