Health and Disease

Down Syndrome : A Genetic Chromosome Disorder

The scope of this article covers the causes of the syndrome, effects, types and finally some lesser-known facts about Down Syndrome!

By Neha Menon

Published 11:30 EST, Tue September 7th, 2021


Most people are born with 46 chromosomes. What are chromosomes? In very simple terms they are structures inside the nucleus of a cell made up of DNA and other proteins. They are typically genetic material and hence they provide a child with genetic and hereditary characteristics. Going back to the first sentence: most people are born with 46 chromosomes. People with Down Syndrome [DS], however, are born with one extra chromosome. This explains why the syndrome is also more commonly called trisomy 21–because in this condition, one is born with an extra copy of their 21st chromosome. The effects of this are commonly a lag in both physical and mental developments. However, the range of these lags or delays can differ from patient to patient and the ability to live with ease also depends. According to the World Health Organization, the predictable incidence of DS is between 1 in 1,000 to 1 in 1,100 live births all over the world.

The scope of this article covers the causes of the syndrome, effects, types and finally some lesser-known facts about Down Syndrome!

Causes of Down Syndrome

As mentioned previously, parents pass on their genes to the child through chromosomes. There is a lot of process and details within this procedure but to understand the causes of the syndrome, it will suffice to know what is mentioned above. Each cell in the child is supposed to have 46 chromosomes, (23 pairs) which are half from the mother and half from the father. 

In situations where the child goes on to develop Down Syndrome, one chromosome cannot separate fully, resulting in 3 copies of the 21st chromosome instead of 2. 

Symptoms of Down Syndrome 

The presence of DS in people typically causes slowed mental and physical development. As mentioned above, this lag or delay in development varies from person to person depending on several factors including the type of DS diagnosed in them (which is discussed in greater detail shortly). Some featured symptoms are (keep in mind that even these differ vastly):

  • Flattened face
  • Small head
  • Short neck
  • Protruding tongue
  • Upward slanting eyelids (palpebral fissures)
  • Unusually shaped or small ears
  • Poor muscle tone
  • Broad, short hands with a single crease in the palm
  • Relatively short fingers and small hands and feet
  • Excessive flexibility
  • Tiny white spots on the coloured part (iris) of the eye called Brushfield’s spots
  • Short height

Some other symptoms are cognitive disabilities such as slowed learning and memory.

Types of Down Syndrome 

There are 3 types of DS: Trisomy 21, mosaicism and translocation. Out of these, the first is the most common. The name is very self-explanatory; trisomy 21 refers to the fact that there is an extra copy of the 21st chromosome in every cell of the body. Mosaicism is very similar, however, it’s different in that not all cells have an extra copy of the 21st chromosome; only some do. Hence there are also lesser symptoms and of lesser intensity in this form. In the 3rd mention form translocation,  there are three 21 chromosomes, but one of the 21 chromosomes is attached to another chromosome. Out of these three given types of Down Syndrome, trisomy-21 is the most prevalent: about 95% of people with DS have this type. 2% of people with DS have mosaic DS and 3% of people with DS have the translocation type. 

Other than the main differences between these 3 types, some other differences that the type of DS can have an influence on are the severity of the symptoms. As mentioned above, every individual’s symptoms vary vastly.

Facts! (

  • The exact cause of the extra chromosome that triggers Down syndrome is unknown.
  • One in every 691 babies in the U.S. is born with Down syndrome, making it the most common chromosomal condition.
  • There are more than 400,000 people living with Down syndrome in the U.S.
  • In 1983, the average life expectancy of a person with Down syndrome was a mere 25-years-old. Today, it is 60.
  • Children and adults with Down syndrome share some common features, but naturally, the individuals will more closely resemble their immediate family members.
  • Since the 1970s, public schools have been required by law to provide free and appropriate education to children with Down syndrome.
  • The likelihood of giving birth to a child with Down syndrome increases with maternal age, however, 80% of babies with Down syndrome are born to women under 35 years of age because this age group gives birth most frequently.
  • Roughly 25% of families in the U.S. are affected by Down syndrome.
  • While behaviour, mental ability, and physical development vary from person to person, many individuals with Down syndrome grow up to hold jobs, live independently, and enjoy normal recreational activities.

Neha Menon, Youth Medical Journal 2021


Team, T. H. (2019, October 29). Down Syndrome: Causes, Types, and Symptoms. Retrieved from

Facts about Down Syndrome. (2021, April 06). Retrieved from

Down syndrome. (2018, March 08). Retrieved from Facts About Down Syndrome. (n.d.). Retrieved from


By Neha Menon

Neha is a student from India who is passionate about psychology and neuroscience and loves researching on these topics. She hopes to build a career around clinical psychology.

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