• Down syndrome, «SIHN drohm», is a condition present at birth that is characterized by intellectual disability and such physical features as upward-slanting eyes; a flat nose; a small head; and short, stubby hands. Down syndrome results from the presence of an extra copy of a particular chromosome. Chromosomes are the parts of a cell that carry genes, which determine hereditary traits and control development. Down syndrome occurs in about 1 of every 800 live births.
    The degree of intellectual disability in children with Down syndrome typically ranges from mild to moderate, although it may be severe in some individuals. Down syndrome may be accompanied by heart disorders, poor vision, and respiratory problems. In addition, the ears and teeth of people with Down syndrome are often small and may be abnormally shaped.
    Down syndrome results from an abnormal number of chromosomes. A typical human body cell has 46 chromosomes. These chromosomes consist of 1 pair of sex chromosomes, which help determine an individual’s sex, and 22 pairs of autosomes (nonsex chromosomes). A person with a Down syndrome has 47 chromosomes, with a trisomy (three copies) of autosome number 21. For this reason, the syndrome is also called trisomy 21.
    Down syndrome results from a defect called nondisjunction, in which chromosome pairs fail to separate during cell division (see Cell [Cell division]). During the normal process of cell division that produces sperm or egg cells in human beings, each chromosome pair is divided between cells. This process results in each sperm or egg cell having one chromosome from each pair, including a single copy of autosome 21. Sperm and egg unite during fertilization, producing an embryo (developing young) with the full complement of pairs. Nondisjunction in autosome pair 21 can result in an egg or sperm cell receiving both copies of the chromosome. If that sperm or egg undergoes fertilization with a normal sperm or egg cell, the resulting child will have three copies and be born with Down syndrome.
    An unborn child’s risk of having Down syndrome increases with the mother’s age, particularly after age 35. However, about 80 percent of children with Down syndrome are born to women under age 35. Genetic experts detect Down syndrome by organizing images of a person’s chromosomes into a profile called a karyotype. A karyotype can also be used to detect Down syndrome in an unborn baby through a procedure called amniocentesis (see Amniocentesis).
    Proper education and care can help children with Down syndrome develop their full potential within the limits of their disability. Children with Down syndrome usually live at home—rather than in specialized care facilities—and attend special classes in public schools. In fact, studies show that children with Down syndrome reared at home generally have a higher IQ and achieve more than those raised in institutions. In the United States, most children with Down syndrome graduate high school. Special workshops offer jobs to adults with Down syndrome, and some adults work in regular, nonspecialized workplaces. However, individuals with severe intellectual disability may require institutional care.
    Down syndrome is named after John Langdon Haydon Down, the British physician who first described the condition in 1866. People once called the condition Mongolism because the facial features of children with the syndrome were thought to resemble those of Asians or Mongols.
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