Syndromes of chromosome number

Some conditions come from having one chromosome too many or too few.

Down syndrome: an extra copy of chromosome 21 (trisomy 21, written 47,+21). It occurs in about 1 in 800 births, usually because chromosomes fail to separate properly when the egg or sperm forms (non-disjunction). The chance rises with the mother's age. People with Down syndrome often have a flat face, slanting eyes, a single crease across the palm, low muscle tone, slower growth and some learning difficulty, and may have heart or hearing problems.

There is no single cure, but speech therapy, physiotherapy and good medical care help a lot. Life expectancy has risen from about 9 years in the early 1900s to 60 or more today.

Klinefelter syndrome: males with an extra X chromosome (47,XXY), about 1 in 1,000 newborn boys. Signs appear mainly from puberty: tall height, small testes, less body hair and some breast growth. A quick test of cells scraped from inside the cheek (a buccal smear) shows a dark spot called a Barr body, an extra, inactive X. Testosterone treatment and counselling help.

Turner syndrome: females with one X missing (45,X), about 1 in 2,500 girls. Signs include short height, a webbed neck, low-set ears, under-developed ovaries and no periods. A buccal smear shows no Barr body. Hormone treatment supports growth and development.

A baby's chromosomes can be checked before birth (prenatal diagnosis) by taking a little of the fluid around the baby (amniocentesis) or of the placenta (chorionic villus sampling).