Down syndrome
Down syndromeDown Syndrome
Down syndrome is a combination of birth defects including some degree of mental retardation and characteristic facial features. It is also called trisomy 21. Down syndrome occurs when there is an abnormality in chromosome 21. It is found in approximately 1 out of 1000 all live births. Each year, 3,000 to 5,000 people are diagnosed with Down syndrome in the United States.
The formal story began in 1866, when a physician named John Langdon Down first described a set of children with common features who were distinct from other children with mental retardation. Down was superintendent of an asylum for children with mental retardation in Surrey, England when he made the first distinction between children who were cretins (later to be found to have hypothyroidism) and what he referred to as "Mongoloids." Down based this unfortunate name on his notion that these children looked like people from Mongolia, who were thought then to have an arrested development. This ethnic insult came under fire in the 1960s from Asian genetic researchers, and the term was dropped from scientific use. Instead, the condition became called "Down's syndrome." In the 1970s, an American revision of scientific terms changed it simply to "Down syndrome," while it still is called "Down's" in Europe.
There are three main types of Down syndrome. The vast majority of children with Down syndrome (approximately 95 percent) have an extra 21 chromosome. Instead of the normal number of 46 chromosomes in each cell, the individual with Down syndrome has 47 chromosomes. This condition is called trisomy 21. The second type is called translocation since the extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. If translocation is found in a child with Down syndrome, it is important to examine the parents' chromosomes, since in at least one-third of the cases, a parent may be a carrier of the translocation. This form of chromosome error is found in 3 - 4 percent of the individuals with Down syndrome. Another chromosome problem, called mosaicism, is noted in about 1 percent of individuals with Down syndrome. In this case, some cells have 47 chromosomes and others have 46 chromosomes. Mosaicism is thought to be the result of an error in cell division soon after conception.
Although individuals with Down syndrome have distinct physical characteristics, generally they are more similar to the average person in the community than they are different. The physical features are important to the physician in making the clinical diagnosis, but no emphasis should be put on those characteristics otherwise. Not every child with Down syndrome has all the characteristics; some may only have a few, and others may show most of the signs of Down syndrome. Some of the physical features in children with Down syndrome include flattening of the back of the head, slanting of the eyelids, small skin folds at the...
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