Down Syndrome, which used to be called mongolism, is a congenital malformation accompanied by moderate to severe mental retardation, is caused by a chromosomal abnormality. People with Down syndrome are often short in height and have a small, round head with a high, flattened forehead and split, dry lips and tongue. A typical feature is a fold of skin, the epicanthic fold, on either side of the bridge of the nose. The palms show a single transverse crease and the soles have a straight crease from the heel to the space between the first and second toes. These people are also subject to congenital heart defects and tend to develop leukemia. Their intelligence quotient, IQ, usually ranges between 20 and 60, but with early intervention and proper education, some people reach higher levels.The overall incidence of Down syndrome is approximately one in 700 births, but the risk varies with the age of the mother. The rate of Down syndrome in children born to 25 year old mothers is approximately 1 in 2000, but the risk increases to 1 in 200 for 35 year olds and more than 1 in 40 for women older than 40 years. Prenatal tests such as amniocentesis and chorionic villus sampling can be used to detect the chromosomal abnormality causing Down syndrome. In addition, maternal blood tests can suggest the presence of a fetus with Down syndrome when levels of alphafetoprotein are lower than usual, or when levels of unpaired estriol and human chorionic gonadotrophin are abnormal.The chromosomal abnormality involved in most cases of Down syndrome is trisomy 21, or the presence of three copies of the 21st chromosome. As a result, the affected person has 47 chromosomes in all body cells instead of the normal 46, although how this causes the condition's symptoms is not yet known. Scientists assume that the reason for the abnormal chromosomal combination is the fertilization of an ovum having 24 chromosomes by a sperm with a normal assortment of 23, but the...
