Page 7 - Down Syndrome
What is Down syndrome and what causes it?
What is Down syndrome?
Introduction-Down syndrome (DS) is a condition in which extra genetic material causes delays in the way a child develops, and often leads to mental retardation. It affects 1 in every 800 babies born.
It is the most common and readily identifiable chromosomal condition associated with mental retardation. It is caused by a chromosomal abnormality: It is the most common genetic cause of severe learning disabilities in children, occurring in one in every 700 infants. Every year, as many as 6,000 babies are born with Down syndrome in the United States. The condition is named after John Langdon Down, the doctor who first identified the syndrome. There's no medical cure for this condition. What causes Down syndrome?
Nothing done before or during pregnancy can cause Down's syndrome. It occurs in all races, social classes and in all countries throughout the world. It can happen to anyone. 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, but not all. 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.
At the time of conception a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from the mother and 23 from the father. In most cases of Down syndrome, however, a child gets an extra chromosome - for a total of 47 chromosomes instead of 46.
Chromosomes: 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 that cell to develop. Genes, which are units of information, are "encoded" in the DNA. Human cells normally have 46 chromosomes which can be arranged in 23 pairs. Of these 23, 22 are alike in males and females; these are called the "autosomes." The 23rd pair are the sex chromosomes ('X' and 'Y').
About 1 to 2 percent of individuals with Down syndrome have a form called mosaicism. In this form, the error in cell division occurs after fertilization. Affected individuals have some cells with an extra chromosome 21 and others with the normal number.
Researchers have established that the likelihood that a reproductive cell will contain an extra copy of chromosome 21 increases dramatically as a woman ages. Therefore, an older mother is more likely than a younger mother to have a baby with Down syndrome. However, of the total population, older mothers have fewer babies; about 75% of babies with Down syndrome are born to younger women.
