Down syndrome occurs because of a chromosomal abnormality present at birth and is the most common cause of intellectual disability (a slower development of thinking and learning skills).  It affects 1 in every 860 live births and is seen in all ethnic groups.

 It gets its name from the 19th Century English physician Dr Langdon Down, who was the first to describe and record common characteristics of people with this condition.  From his name we get the word "Down".  The word "syndrome" means a group of signs or symptoms that occur together to characterise a particular condition; hence Down syndrome.

In 1959 French doctor, Jerome Lejeune, discovered that people with Down syndrome have an extra chromosome.  This relatively recent discovery is very important for medical and social reasons, as it allowed medical researchers, parents and the general public to have accurate information about the condition and its implications.

How does Down syndrome occur?

People unaffected by any chromosomal conditon are born with 46 chromosomes in each cell (22 pairs plus 2 sex chromosomes).  After conception, the fertilised egg begins to divide.  Each new cell will have a matching set of chromosomes and each cell will therefore, have the same genetic code as the first cell.  In Down syndrome the chromosomes in the very early cells do not pair off correctly, and the baby begins to develop with 47 chromosomes.  As a result of the extra chromosome, 1 of 3 types of Down syndrome can occur:

Trisomy 21

This is the most common form of Down syndrome, affecting about 94% of people with the syndrome.  Here, the extra chromosome aligns itself with the 21st pair of chromosomes (hence Trisomy 21).  Although we do know that Trisomy 21 is not hereditary, we do not know what causes this particular cell division.

Translocation Down syndrome

In 3-4% of cases Down Syndrome is due to the presence of the long arm, rather than the whole, of an extra chromosome 21.  The long arm of the extra chromosome 21 is attached to another chromosome, often 13,14,15,21 or 22.  This process is often referred to as a Robertsonian Translocation.  A person with Translocation Down syndrome will have cells containing 46 chromosomes, but will have the features associated with Down Syndrome because the extra genetic material from Chromosome 21 gets stuck or `translocated` onto another chromosome.  As far as we know, people with translocation are generally affected to a similar extent as with Trisomy 21. 

The majority of translocations are random events, and parents will have no greater chance of having a baby with Down syndrome than others.  One in three translocations are inherited and a chromosome test for both parents is usually carried out as it is possible for one parent to be a `balanced` carrier of Trisomy 21. 

Mosaic Down syndrome

About  2-3% of people with Down syndrome have Mosaicism.  Here, some cells divide incorrectly while others will continue to reproduce normally.  This produces a "mosaic" pattern or a mixture of cells, where some have 46 chromosomes and some have 47 (as in Trisomy 21).  The proportion of "normal" cells and Trisomy 21 cells will vary greatly from individual to individual and for this reason, people with Mosaicism may be affected by the syndrome to a greater or lesser extent than those with Trisomy 21.  Mosaicism is not hereditary. 

How do we determine which form of Down syndrome our child has?

When a child is born with Down syndrome, or if Down syndrome is suspected, a chromosomal analysis is carried out to confirm the diagnosis, and to determine which type of Down syndrome the child has.  This testing is usually carried out shortly after birth by means of blood tests.

Characteristics of Down syndrome

Although children with Down syndrome share some of the same characteristics and may look alike, each child is different and will also look like their parents and family.  Each person with Down syndrome is a unique individual just as all persons are unique individuals and can vary enormously in appearance, temperament and ability.  Approximately 98% of people with Down syndrome have an intellectual disability.  Most children will have a moderate intellectual disability. Medical researchers have identified more than 50 characteristics which may be found in people with Down syndrome.  Most people with Down syndrome will have only a few of these. 

The more common characteristics include:

• eyes that may have a slant an an extra fold of skin on the inner side (epicanthal fold)
• small white flecks on the iris (Brushfield spots)
• a small, flatter nose
• smaller and/or shorter limbs, fingers, hands, toes and feet
• a single crease across the palm of the hand
• congenital heart defects (defects formed before birth) are found in approximately 50% of children with Down syndrome.
• low muscle tone and loose ligaments (especially in the hips)
• a range of associated medical conditions including hearing and vison problems and reduced immune function.

Articles and Information

Trisomy 21 : The Story of Down syndrome by Len Leshin

Down Syndrome Clearly Explained

Facts about Down Syndrome

Basic Cell Biology for Parents of a Child with Down Syndrome by Ginger Houston-Ludlam

Resources

Selikowitz, Mark  Down Syndrome : the facts

Stray-Gundersen  Babies with Down Syndrome : a new parent's guide

Medical and Surgical Care for Children with Down Syndrome

Schermerhorn, Will  Down Syndrome : the first 18 months (DVD+VHS)

Links

Genetics Services of WA (GSWA)

Diagnosis, counselling and family assessment of children with intellectual disability

Clinic: PMH level 6 Hay St Building, Roberts Rd Subiaco 6008

Phone: (08) 9340 1625

Email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it