What is Down syndrome

What is Down Syndrome?

Down syndrome is named after the English physician John Langdon Down, who characterised the condition, but did not have it.

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.

There are three types of Down syndrome: trisomy 21 (nondisjunction) accounts for 95% of cases, translocation accounts for about 4%, and mosaicism accounts for about 1%.

Down syndrome is the most commonly occurring chromosomal condition.

Down syndrome occurs in people of all races and economic levels. The incidence of births of children with Down syndrome increases with the age of the mother. But due to higher fertility rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age.

People with Down syndrome have an increased risk for certain medical conditions such as congenital heart defects, respiratory and hearing problems, Alzheimer’s disease, childhood leukemia and thyroid conditions. Many of these conditions are now treatable, so most people with Down syndrome lead healthy lives.

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.

Every person with Down syndrome is a unique individual and may possess these characteristics to different degrees or not at all.

Life expectancy for people with Down syndrome has increased dramatically in recent decades – from 25 in 1983 to 60 today. People with Down syndrome attend school, work, participate in decisions that affect them, have meaningful relationships, vote and contribute to society in many wonderful ways.

All people with Down syndrome experience cognitive delays, but the effect is usually mild to moderate and is not indicative of the many strengths and talents that each individual possesses.

Quality educational programs, a stimulating home environment, good health care and positive support from family, friends and the community enable people with Down syndrome to lead fulfilling and productive lives.

 

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.

MOSAICISM

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.

TRANSLOCATION

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.