Down's syndrome karyotype. Artwork based on a light micrograph of the chromosomes of a person affected by Down's syndrome. Down's syndrome (trisomy-21) is caused by a chromosomal anomaly: the affected person has three copies of chromosome 21 in their cells rather than the normal two. The extra chromosome is shown green here, and highlighted with yellow (lower center). The defect results in mental retardation and physical disability, including a flattened face and an increased risk of infection and neck damage. The condition is more prevalent in the children of older mothers.