The Causes of Down’s Syndrome

Down syndrome also known as Trisomy 21 is due to a random error in the Cells of the individuals. This error occurs in the genetic material of the cells. This genetic material known as chromosomes helps code for such things as eye colour, hair colour and height (4). Most people have 46 chromosomes in their cells but people with Down’s Syndrome contain an extra copy chromosome of 21, hence the name Trisomy 21 (6).

 

Down’s Syndrome usually occurs as a random change when cells multiply, research as shown that in 90% of cases the extra chromosome comes from the mother, in 4% of cases from the father and 6% of cases this error arises after the baby is conceived (5).

 

There are three main types of Down’s Syndrome: Complete, Mosaic and Translocation.

 

Complete Down’s Syndrome, is also known as Nondisjunction, occurs when there is an error during the formation of the mother’s egg or the father’s sperm (5). This results in the baby having a full extra copy of chromosome 21. Within in the cells the baby contains three full copies of chromosome 21 opposed to the normal 2 copies. As the baby grows the extra chromosome is present within every cell of the body (4). This accounts for 95% of all cases of Down’s Syndrome, therefore is the most common type (5).

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Diagram showing how nondisjunction occurs – (Pediactrics Clerkship, 2017) (7)

Mosaic Down’s Syndrome, occurs in 1% of cases (5). Each cell in the body is different, there is an assortment of normal cells and cells containing the information for Down’s Syndrome (6).  This occurs when as an error in early development it is either due to some cells losing a chromosome as the baby develops or incorrect division of cells as a baby develops (5). Some research is indicated that individuals with Mosaic Down’s Syndrome may exhibit less characteristics of Down’s Syndrome in comparison. This is the least common form of Down’s Syndrome (6).

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Diagram showing Mosaicism concept – (Bioninja, 2017) (1)

Translocation Down’s Syndrome, occurs when the cells of the individual contain only a part of the extra chromosome 21 (5). This extra piece of chromosome gets attached to another chromosome within the individuals’ cells which is then passed on as cells divide (6). This occurs in about 4% of Down’s Syndrome cases, and there is little to no difference between people with this type of Down’s Syndrome and people with complete Down’s Syndrome (5).

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Diagram showing concept of translocation – Childrens Hospital of Wisconsin (2017). (2).

The cause Down’s Syndrome is mostly unknown (6). However, it has been attributed to increasing maternal age, for example a woman in her 20s has a 1/1500 chance of having a baby with Down’s Syndrome compared to a woman in her 40s who has a 1/100 chance of having a baby with Down’s Syndrome (4). There is no significant evidence which may hint that environmental factors or parental activity during pregnancy attributes to Down’s Syndrome (6).

 

Down’s Syndrome is a genetic condition but despite this fact only 1% of cases are passed from mother to daughter/father to son, and this is only the case in one type of Down’s Syndrome which is translocation (6).

Article by: Caitlin Dollin

References:

  1. Bioninja (2017). Concept of Mosaicism. [image] Available at: http://ib.bioninja.com.au/_Media/Mosaicism_med.jpeg [Accessed 9 Dec. 2017].
  2. Childrens Hospital of Wisconsin (2017). Translocation Concept. [image] Available at: https://www.chw.org/-/media/images/medicalcare/healthinformation/chromosome_d.gif?la=en&hash=011039023FCD9E9B051B32E9DC0ADE65B8CF412 [Accessed 9 Dec. 2017]
  3. National Down Syndrome Society (2017). Maternal Age Chart. [image] Available at: http://www.ndss.org?wp-content/uploads/2017/08/Maternal-Age-Chart.jpg [Accessed 9 Dec. 2017].
  4. nhs.uk. (2017). Down’s syndrome. [online] Available at: https://www.nhs.uk/conditions/downs-syndrome/#causes-of-downs-syndrome [Accessed 7 Dec. 2017].
  5. Nichd.nih.gov. (2017). What causes Down syndrome?. [online] Available at: https://www.nichd.nih.gov/health/topics/down/conditioninfo/pages/causes.aspx [Accessed 7 Dec. 2017].
  6. NDSS. (2017). What is Down Syndrome? | National Down Syndrome Society. [online] Available at: http://www.ndss.org/about-down-syndrome/down-syndrome/ [Accessed 7 Dec. 2017]
  7. Pediatrics Clerkship (2017). Nondisjunction. [image] Available at: https://pedclerk.bsd.uchicago.edu/sites/pedclerk/uchicago.edu/files/uploads/non-disjunction-or-aneuploidy_1.jpeg [Accessed 9 Dec 2017].