According to the National Down Syndrome Association, In every cell of our bodies there is a nucleus, where genetic material is stored. These genes carry codes responsible for all of our inherited traits, they are grouped among our chromosomes. Typically the nucleus of each cell contains 23 pairs of chromosomes, half from your mother and half from your father. Down syndrome occurs when an individual has an extra full or partial copy of chromosome 21.
This additional chromosome alters development and causes characteristics associated with Down syndrome. A few of the common physical traits of Down syndrome are:
- low muscle tone
- small stature, an upward slant to the eyes
- a single deep crease across the center of the palm
In the late nineteenth century John Langdon Down, an English physician, published an accurate description of a person with Down Syndrome. His scholarly work published in 1866, earned him the recognition as the "father" of the syndrome.
In 1959, the French physician Jérôme Lejeune identified Down syndrome as a chromosomal condition. Instead of the usual 46 chromosomes present in each cell, Lejeune observed 47 in the cells of individuals with Down syndrome. It was later determined that an extra partial or whole copy of chromosome 21 results in the characteristics associated with Down syndrome. In the year 2000, an international team of scientists successfully identified and catalogued each of the approximately 329 genes on chromosome 21. This accomplishment opened the door to great advances in Down syndrome research.
Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.
The cause of nondisjunction is currently unknown, but research has shown that it increases in frequency as a woman ages. However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age.
There is no definitive scientific research that indicates that Down syndrome is caused by environmental factors or the parents' activities before or during pregnancy.
Down syndrome occurs in people of all races and economic levels, though older women have an increased chance of having a child with Down syndrome. A 35 year old woman has about a one in 350 chance of conceiving a child with Down syndrome, and this chance increases gradually to 1 in 100 by age 40. At age 45, the incidence becomes approximately 1 in 30. The age of the mother does not seem to be linked to the risk of translocation.
Since many couples are postponing parenting until later in life, the incidence of Down syndrome conceptions is expected to increase. Therefore, genetic counseling for parents is becoming increasingly important. Still, many physicians are not fully informed about advising their patients about the incidences of Down syndrome, advancements in diagnosis, and the protocols for care and treatment of babies born with Down syndrome.
All 3 types of Down syndrome are genetic conditions (relating to the genes), but only 1% of all cases of Down syndrome have a hereditary component (passed from parent to child through the genes). Heredity is not a factor in trisomy 21 (nondisjunction) and mosaicism. However, in one third of cases of Down syndrome resulting from translocation there is a hereditary component - accounting for about 1% of all cases of Down syndrome.
The age of the mother does not seem to be linked to the risk of translocation. Most cases are sporadic - chance - events. However, in about one third of cases, one parent is a carrier of a translocated chromosome.
For more information on Down Syndrome visit the National Down Syndrome Society