From the Mayo website

As promised, here are some interesting facts about Down syndrome...

Children with Down syndrome have a distinct facial appearance. Though not all children with Down syndrome have the same features, some of the more common features are:

 Flattened facial features
 Protruding tongue
 Small head
 Upward slanting eyes, unusual for the child's ethnic group
 Unusually shaped ears

Children with Down syndrome may also have:
 Poor muscle tone
 Broad, short hands with a single crease in the palm
 Relatively short fingers
 Excessive flexibility

Infants born with Down syndrome may be of average size, but typically they grow slowly and remain shorter than other children of similar age. Children with Down syndrome also have some degree of mental retardation, most often in the moderate range.

Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from your father, the other from your mother.

The cause of Down syndrome is one of three types of abnormal cell division involving chromosome 21. All three abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome. The three genetic variations that can cause

Down syndrome include:

 Trisomy 21. More than 90 percent of cases of Down syndrome are caused by trisomy 21. A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells. This form of Down syndrome is caused by abnormal cell division during the development of the sperm cell or the egg cell.
 Mosaic Down syndrome. In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
 Translocation Down syndrome. Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. Children with translocation Down syndrome have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 stuck to the translocated chromosome. This form of Down syndrome is uncommon.
There are no known behavioral or environmental factors that cause Down syndrome.

Is it inherited?

Most cases of Down syndrome aren't inherited. They're caused by a mistake in cell division during the development of the egg, sperm or embryo.

Translocation Down syndrome is the only form of the disorder that can be passed from parent to child. However, only about 4 percent of children with Down syndrome have translocation. And only about half of these cases are inherited from one of the parents.

In these translocation cases, the mother or father is a balanced carrier of the translocation, which means he or she has some rearranged genetic material, but no extra genetic material. A balanced carrier has no signs or symptoms of Down syndrome, but he or she can pass the translocation on to children.

The chance of passing on the translocation depends on the sex of the parent who carries the rearranged chromosome 21:

 If the father is the carrier, the risk is about 3 percent.
 If the mother is the carrier, the risk is about 12 percent.
Children with Down syndrome can have a variety of complications, including:
 Heart defects. Approximately half the children with Down syndrome are born with some type of heart defect. These heart problems can be life-threatening and may require surgery in early infancy.
 Leukemia. Young children with Down syndrome are more likely to develop leukemia than are other children.
 Infectious diseases. Because of abnormalities in their immune systems, those with Down syndrome are much more susceptible to infectious diseases, such as pneumonia.
 Dementia. Later in life, people with Down syndrome have a greatly increased risk of dementia. Signs and symptoms of dementia often appear before age 40 in people with Down syndrome.
 Sleep apnea. Because of soft tissue and skeletal alterations that lead to the obstruction of their airways, children with Down syndrome are at greater risk of obstructive sleep apnea.
 Obesity. People with Down syndrome have a greater tendency to be obese than does the general population.
 Other problems. Down syndrome may also be associated with other health conditions, including gastrointestinal blockage, thyroid problems, hearing loss, skeletal problems and poor vision.

Life expectancy

Life spans have increased dramatically for people with Down syndrome. In 1929, a baby born with Down syndrome often didn't live to age 10. Today, someone with Down syndrome can expect to live to 50 and beyond, depending on the severity of his or her health problems. Life span continues to increase because of early interventions and better care.