A chromosomal abnormality resulting in a variable degree of learning difficulties and a characteristic physical appearance. People with Down’s syndrome have an extra chromosome (47 instead of 46). Affected individuals have 3 copies of chromosome number 21 instead of 2; the disorder is also called trisomy 21. In most cases, it is the result of a sperm or egg being formed with an extra chromosome 21. If one of these takes part in fertilization, the baby will also have the extra chromosome. This type of abnormality is more likely if the mother is aged over 35. A less common cause is a chromosomal abnormality known as a translocation, in which part of 1 parent’s own chromosome number 21 has joined with another chromosome. The parent is unaffected but has a high risk of having Down’s children. Typical physical features of a person with Down’s syndrome include small face and features; sloping eyes with folds of skin that cover their inner corners; large tongue; and short, broad hands. People with Down’s syndrome have a greater than normal risk for certain disorders, such as a heart defect at birth (see heart disease, congenital), intestinal atresia (a narrowing in the intestines), congenital deafness, and acute leukaemia. Down’s syndrome children are especially susceptible to ear infections. A type of Alzheimer's disease often develops after age 40. Down’s syndrome is usually recognized soon after birth. The diagnosis is confirmed by chromosome analysis. Screening tests during early pregnancy, including ultrasound scanning, indicate those fetuses likely to have the syndrome. Amniocentesis is then offered.
Down’s syndrome |
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