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 female with trisomy 18. Humans have 46 chromosomes arranged in 23 pairs. Trisomy 18 is caused by three copies of chromosome 18; a female has two X sex chromosomes. Trisomy 18 causes severe deformity and mental retardation, and few affected infants live over a year.
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