HERVE CONGE, ISM / SCIENCE PHOTO LIBRARY HERVE CONGE, ISM / SCIENCE PHOTO LIBRARY
Light micrograph of the chromosomes (karyotype) of a patient 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. Down's syndrome results in mental retardation and flattened facial features.
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