|What is Down Syndrome?|
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:
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.
What is translocation Down syndrome?
Translocation Down syndrome refers to the rearranged chromosome material. There are three # 21 chromosomes, just like there are in trisomy 21, but the long arm of one of the 21 chromosomes is attached to another chromosome, instead of being separate. The short arm of the broken chromosome does not survive. The extra #21 chromosome material is what causes the problems that make up Down syndrome. In translocation Down syndrome, the extra 21 chromosome may be attached to the #14 chromosome, or to #s13, 15, or 22. In some cases, two #21 chromosomes can be attached to each other.
Three to four percent of babies born with Down syndrome have translocation Down syndrome. If you had a roomful of 100 babies with Down syndrome, they would all look alike and have similar features and health problems. You would not be able to pick out the three or four that have a translocation.
Whenever a translocation is found in a child, the parents' chromosomes are studied to determine whether the translocation was inherited or not. If one parent has the translocation chromosome, then the physician knows the baby inherited the translocation from that parent. The parent will actually have 45 total chromosomes in each cell of their body, but they will be normal and healthy because they still have only two copies of each cbromosome. When a person has a rearrangement of chromosome material, with no extra or missing chromosome material, they are said to have a 'balanced translocation' or be a 'balanced translocation carrier.'
Parents with balanced translocations may have fertility problems (trouble becoming pregnant), miscarriages, or have an increased chance of having a child with health problems. Although the parent can donate the proper amount of genetic material (23 chromosomes ) to a pregnancy, they have a risk of donating too much or too little genetic material to a pregnancy. It is not something they can control or predict. The chance depends on the type of chromosome rearrangement and which chromosomes are involved. For example, if the translocation is between #14 and 21, there is a 10 to 15 percent chance for Down syndrome in a pregnancy if the mother is the translocation carrier, and a 3 to 5 percent chance if the father is the translocation carrier. The chances are different for men and women because sperm and egg are produced differently. A woman is born with all of the eggs she will ever have while men are constantly producing new sperm.
There is another important factor to remember when a parent is found to have a translocation. The parents' relatives (brothers, sisters) may also have inherited the translocation and, therefore, may have the same risks for problems with a pregnancy. For these reasons, it is recommended that people with chromosome rearrangements share this information with their relatives so that they can heave the option of having their chromosomes studied.
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:
Articles and Information
Trisomy 21 : The Story of Down syndrome by Len Leshin
Basic Cell Biology for Parents of a Child with Down Syndrome by Ginger Houston-Ludlam
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)
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