Down Syndrome (DS)

Down Syndrome (DS) was named after Dr. John Langdon Down, the British doctor who first identified the characteristics back in 1866.  However, it was not until 1959 that Dr. Jerome Lejeune of Paris linked the cause of DS to chromosomes.  Dr. Lejeune found that where most of us have 46 chromosomes made up of 23 pairs, people with Down Syndrome have three number 21 chromosomes.  

Though all people with DS have extra chromosomes, there are three different types of DS:

Trisomy 21 - the most common making up about 95% - have an extra chromosome 21 in all of their cells.

Translocation - making up about 4%  - has the extra chromosome 21 material in part of another chromosome.

Mosaicism - found in only about 1% of  individuals with DS - finds an extra whole chromosome is present, but only in some of their cells. This makes the physical symptoms a lot less obvious as many of their cells are normal.

Doctors are not exactly certain why some cells are normal and others are not but it is known that most cases of DS are not inherited, parents do not cause it, nor can it be prevented .  

People with Down Syndrome often have speech problems, heart defects and respiratory problems.  Many have other physical characteristics such as upward slanted eyes, small ears and noses and are often shorter than people their age.  Compared to other children, children with Down Syndrome are usually slower to learn.