Down Syndrome

Did you know there is at least one case of Down syndrome in every 700 pregnancies? Research also suggests that the older a mother, the more likely she is to give birth to a child with Down syndrome. However, there are several other factors which are also deemed responsible for giving rise to this disorder. Understandably, this turns out to be a vital topic which must be understood by every student of Biology.

Read along to find more about the Down syndrome symptoms, features and types!

What is Down Syndrome?

The Down syndrome definition describes it as a chromosomal disorder, wherein, an error in cell division leads to an extra copy of chromosomes in the body. This extra chromosome 21 tends to affect the physical and cognitive abilities of affected individuals.

Additionally, individuals with Down syndrome often run a higher risk of suffering from various health disorders.  Notably, it is not a disease rather a term that best describes the features which arise due to it. 

On that note, let us proceed to find types of Down syndrome and their differences.

Types of Down Syndrome

Typically, there are 3 types of Down syndrome –

1. Trisomy 21

It is the most common of the three types and accounts for almost 95% of the cases. It is mostly caused by the irregular separation of chromosome 21 during meiosis either in egg cells or sperm. When the fertilized egg develops, it transfers an extra copy of this chromosome to each cell in the body.

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Fig: Trisomy 21

2. Translocation 

It accounts for 4% of the total cases. It must be noted that in this particular type, the extra chromosome is transferred to new cells differently. 

It occurs only when a significant part of chromosome 21 breaks off amidst the replication process and gets attached to another chromosome. This leads to a cell with 46 chromosomes and a little extra chromosome 21. 

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Fig: Translocation

3. Mosaicism 

This particular type accounts for as little as 1% of the total cases. Notably, in this form, the error in separation takes place right after the egg is fertilized.

The role of timing is the most noteworthy here. Also, the fact that not all cells have an extra chromosome makes the symptoms of mosaic Down syndrome less prominent.

Do It Yourself: Define Down syndrome and mention its types. 

Down Syndrome Features 

These are some Down syndrome features –

A. Physical Features

  • Small stature with a short neck

  • Flat nasal bridge

  • low muscle tone

  • Eyes slant upwards

  • Eyes have an additional fold on the upper eyelid

  • A large gap between the large and second toe

B. Cognitive Features

  • Intellectual disability ranges from mild to moderate level

  • Delay in learning to speak

  • Delay in motor skills

  • Problematic attention span

Other than these, individuals with Down syndrome are prone to health problems like – congenital heart defect, epilepsy, respiratory disorders, hearing difficulty, leukaemia, Alzheimer’s, thyroid, etc. 

One can diagnose this anomaly through a screening test and other prescribed diagnostic tests. 

This image below highlights the Down syndrome symptoms.

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Fig: Down syndrome symptom diagram

Test Your Knowledge: Highlight the differences between the 3 types of Down syndrome.

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FAQ (Frequently Asked Questions)

1. What is Down Syndrome Definition?

Ans.  According to Down syndrome definition biology, it is a chromosomal anomaly in which an error in cell division leads to an extra copy of chromosome 21 in the body. It is divided into 3 types – Trisomy 21, Mosaic Down syndrome and Translocation. 

2. What are Down Syndrome Symptoms?

Ans. Most common physical attributes include around and flat face, eyes that slant upwards, nose with a low bride, smaller mouth cavity, etc. Also, they may show signs of mild to moderate cognitive disabilities. 

3. How is Down Syndrome Diagnosed?

Ans. It is often diagnosed immediately after birth by judging the child’s clinical features. However, it is only confirmed post blood test. Similarly, prenatal tests often help to detect the case of down syndrome effectively.