PR. PHILIPPE VAGO, ISM / SCIENCE PHOTO LIBRARY PR. PHILIPPE VAGO, ISM / SCIENCE PHOTO LIBRARY
Light micrograph of the chromosomes (karyotype, with G banding) of a male with Down's syndrome. Humans have 46 chromosomes arranged in 23 pairs. Down's syndrome (trisomy 21) is caused by three copies of chromosome 21; a male has one X and one Y sex chromosomes. Down's syndrome results in mental retardation and flattened facial features.
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