Seeach Sod - Defining Down Syndrome

Defining Down Syndrome

“By Definition…”

By: Dr. M. Weil,

Chief physician at Seeach Sod,

Developmental pediatrician and pediatric specialist

Down Syndrome; Is genetic predisposition a factor in this syndrome?

Down syndrome is the most common chromosomal disorder and is caused by the presence of an extra 21^st chromosome instead of the usual pair (hence it’s other name “trisomy 21”). A person with Down syndrome has a total of 47 chromosomes in each cell instead of 46.

In most cases (about 95%), there is full trisomy, meaning 3 complete 21^st chromosomes. Maternal age can impact the occurrence of trisomy 21, which is the result of irregular cell division. Women over the age of 35 are at a higher risk of giving birth to a Down syndrome child than their younger counterparts, although there are incidences of Down syndrome children born to young mothers. This condition is not hereditary and is unrelated to the parents’ chromosomal chemistry.

Another form of Down syndrome is one which is called mosaic Down syndrome. In such a condition, some of the cells in the body are normal while other cells have trisomy 21. Mosaic Down syndrome is manifested as a much milder form of the disorder and usually occurs as a result of irregular cell division and a partial correction of the abnormal division.

Heredity plays no role in both of these conditions and family members of individuals with Down syndrome are at no greater risk of giving birth to a child with Down syndrome.

In about 3-4% of individuals with Down syndrome, the disorder is due to a chromosomal translocation in one of the parents. In this case, chromosome 21 is attached to another chromosome, often chromosome 14. A person with such a translocation has no signs of clinical Down syndrome and is completely normal (they don’t have an extra chromosome; rather they have one less, 45, because one chromosome 21 is fused with another chromosome). However, carriers of chromosomal translocation have a greater chance of giving birth to a child with Down syndrome.

It is therefore important that individuals with Down syndrome and their parents undergo chromosomal testing, in order to verify whether they are carriers of translocation.