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Down Syndrome: A genetic disorder that is caused by the presence of an extra chromosome, which results in varying degrees of physical and mental abnormality. Physical characteristics of the syndrome include a flattened face, widely spaced and slanted eyes, comparatively smaller head size and lax joints. Although not present in all cases, mental retardation is typical. Those who are affected with the syndrome display a wide variety in mental, behavior and developmental capabilities, and will have a high propensity to suffer from common health problems, include a low resistance to infection, pronounced hearing loss, gastrointestinal problems, and heart defects.

Down syndrome is usually caused by an error in cell division called non-disjunction. There are also two other types of chromosomal abnormalities (mosaicism and translocation) that to a lesser extent are also implicated as possible causes of Down syndrome. Regardless of the type of Down syndrome which a person may have, all people with Down syndrome have an extra, critical portion of the number 21 chromosome present in all or some of their cells. The presence of this additional genetic material alters the normal course of development, which results in the development of the characteristics that are normally associated with the syndrome.

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The definition of this term has been taken from the "Adoption Glossary," which contains definitions of a wide variety of words that relate to adoption and infertility. The entire Adoption Glossary is located at http://glossary.adoption.com.

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