Trisomy Health Dictionary

The presence of an extra chromosome within a person’s cells, making 3 of a particular chromosome instead of the usual 2. A fault during meiosis to form egg or sperm cells leaves an egg or sperm with an extra chromosome. When the egg or sperm takes part in fertilization, the resulting embryo inherits an extra chromosome in each of its cells.

The most common trisomy is of chromosome 21 (Down’s syndrome). Trisomy 18 (Edward’s syndrome) and trisomy 13 (Patau’s syndrome) are less common; trisomy 8 and trisomy 22 are very rare. Partial trisomy, with only part of a chromosome in triplicate, also occurs. Full trisomies cause abnormalities such as skeletal and heart defects and learning difficulties. Except in Down’s syndrome, babies usually die in early infancy. The effects of partial trisomies depend on the amount of extra chromosomal material present.

Diagnosis is made by chromosome analysis of cells, which may be obtained from the fetus by amniocentesis or after the birth. There is no specific treatment. Parents of an affected child should seek genetic counselling.

n. a condition in which there is one extra chromosome present in each cell in addition to the normal (diploid) chromosome set. A number of chromosome disorders are due to trisomy, including *Down’s syndrome and *Klinefelter’s syndrome. —trisomic adj.