SOVEREIGN , ISM / SCIENCE PHOTO LIBRARY SOVEREIGN , ISM / SCIENCE PHOTO LIBRARY
Light micrograph of the chromosomes (karyotype) of a female with Down's syndrome (mongolism). Humans have 46 chromosomes arranged in 23 pairs. Down's syndrome (trisomy 21) is caused by three copies of chromosome 21; a female has two X sex chromosomes (at bottom right). Down's syndrome results in mental retardation and flattened facial features.
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