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Down Syndrome

Down syndrome

In every cell in the human body there is a nucleus, where genetic material is stored in genes.  Genes carry the codes responsible for all of our inherited traits and are grouped along rod-like structures called chromosomes.  Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21.

This additional genetic material alters the course of development and causes the 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, and a single deep crease across the center of the palm – although each person with Down syndrome is a unique individual and may possess these characteristics to different degrees, or not at all.

Down syndrome commonly referred to as `Trisomy 21′ is a chromosomal condition in which there is generally an extra copy of the 21st chromosome. This syndrome usually, though not always, results in a person with friendly, sociable disposition, a mental handicap and other conditions.

Down syndrome is a genetic condition that results in slower physical and intellectual development. It is one of the most common genetic conditions, affecting approximately 1 out of every 660 births, occurring irrespective of race, religion and socio-economic status. People with

Down syndrome have 47 chromosomes instead of the usual 46. Chromosomes are thread like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for a cell to develop.

People with Down syndrome look remarkably similar, regardless of race or color. Their heads are often small and shaped slightly differently from people with 46 chromosomes. Their special facial features include almond-shaped eyes, a protruding tongue and a flattened nose. Their hands are usually small and broad with broad fingers and they are often short and stocky in build.

Parents who have already had a baby with Down syndrome, mothers or fathers who have a re-arrangement involving chromosome 21, and mothers over 35 years of age are most likely to have a baby with Down syndrome. The chance of having a baby with Downs increases with age, from about 1 to 1250 for a woman at age 25, to 1 – 1000 at the age 30, to 1-400 at the age 35 and 1-100 at the age 40.

Are There Different Types of Down syndrome?

There are three types of Down syndrome:  trisomy 21 (nondisjunction), translocation and mosaicism.

Trisomy 21 (Nondisjunction)

Down syndrome is usually caused by an error in cell division called “nondisjunction.”  Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two.  Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.  As the embryo develops, the extra chromosome is replicated in every cell of the body.  This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.


In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.


Mosaicism (or mosaic Down syndrome) is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47.  Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism is the least common form of Down syndrome and accounts for only about 1% of all cases of Down syndrome.  Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.  However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.

Signs of Down syndrome:

  • Low muscle tone at birth
  • Separated sutures (joints between the bones of the skull)
  • Asymmetrically shaped skull
  • Round head with flat area at the back of the head
  • Small skull
  • Almond-shaped eyes, unusual for ethnic group
  • Small mouth with protruding tongue
  • Broad short hands
  • Single crease on palms
  • Slow growth and development, with the gap between chronological age and expected maturation steadily increasing
  • Brushfield’s Spots of the iris (an unusual spot in the colored part of the eye)

Does Down syndrome Run in Families?

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 compontent – 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.

What Impact Does Down syndrome Have on Society?

Individuals with Down syndrome are becoming increasingly integrated into society and community organizations, such as school, health care systems, work forces, and social and recreational activities. Individuals with Down syndrome possess varying degrees of cognitive delays, from very mild to severe. Most people with Down syndrome have cognitive delays that are mild to moderate.

Due to advances in medical technology, individuals with Down syndrome are living longer than ever before. In 1910, children with Down syndrome were expected to survive to age nine. With the discovery of antibiotics, the average survival age increased to 19 or 20. Now, with recent advancements in clinical treatment, most particularly corrective heart surgeries, as many as 80% of adults with Down syndrome reach age 60, and many live even longer. More and more Americans are interacting with individuals with Down syndrome, increasing the need for widespread public education and acceptance.