What is down syndrome? • Down syndrome (DS), is a genetic condition in which a person has 47 chromosomes instead of the usual 46. • The disorder causes mental retardation and other problems. • The physical features and medical problems associated with Down syndrome can vary widely from child to child. While some kids with DS need a lot of medical attention, others lead healthy lives. • Though Down syndrome can't be prevented, it can be detected before a child is born. The health problems that can go along with DS can be treated, and there are many resources within communities to help kids and their families who are living with the condition.
Understanding Chromosomes • Chromosomes, the spaghetti-like strands of hereditary material found in each cell of the body, determine such characteristics as the color of our eyes and hair, our height, and whether we are male or female. In humans, there are 23 pairs of chromosomes in each cell. A person inherits one copy of each chromosome from each parent.
Understanding Chromosomes • Each cell contains 22 pairs of chromosomes (called autosomes) that are the same in males and females. The remaining pair of chromosomes, the X- and Y-chromosomes, are not shaped similarly, and thus are not matched in the same way as the autosomes. The X- and Y-chromosomes are called sex chromosomes. They are responsible for the difference in development between males and females. A Y-chromosome contains genes responsible for testis development; and the presence of an X-chromosome paired with a Y-chromosome will determine male development. On the other hand, two X-chromosomes are required for normal ovarian development in females.
Extra Chromosome 21 • Three genetic variations can cause Down syndrome. Most cases of Down syndrome (92 percent) result from trisomy 21, which means that each cell in the body has three copies of chromosome 21 instead of the usual two copies. • The cause of Down syndrome in 2 to 4 percent of cases occurs when only some of the body's cells have an extra copy of chromosome 21. These cases are called mosaic Down syndrome.
What Causes It? • The causes of Down syndrome are genetic problems related to chromosome 21. While Down syndrome is genetic in that it involves an extra chromosome 21, it is not usually hereditary in the conventional sense. That is, Down syndrome is not inherited; it does not "run in families." Down syndrome is usually the result of a random event that occurs during the formation of reproductive cells (eggs and sperm). One exception is Down syndrome caused by a balanced translocation. • There are 3 possible things which can result in extra genetic material from chromosome 21 which causes down syndrome.
Trisomy 21 • More than 92 percent of cases with down syndrome are caused by trisomy 21. • With trisomy 21 there are 3 copies of chromosome 21 instead of usual 2 copies.
2. Mosaic down syndrome • Rare form of down syndrome • Some cells have extra copy of chromosome 21, but not all. • Is caused by abnormal cell division after fertilization.
3. Translocation down syndrome • Occurs when a part of chromosome 21 becomes attached onto another chromosome before or at conception. • Children with translocation have the usual two copies of chromosome 21, but have additional material from chromosome 21 stuck to the translocated chromosome.
Is Down Syndrome Inherited? • Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). The one exception is some cases of translocation Down syndrome. • In these cases, an unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.