Karyotype showing the arrangement of chromosomes in a female with Down's syndrome (trisomy-21), the most common diagnosable cause of mental handicap, computer illustration. Down's syndrome is caused by a chromosomal anomaly: the 21st set having three rather than the normal two chromosomes. In karyotyping, chromosomes are arranged in numbered pairs according to a standard classification. The female set differs from the male in the sex pair (bottom right); a male would have XY rather than XX. A female Down's karyotype is written: 47,XX,+21. The normal chromosome count is 46, 23 from maternal and 23 from paternal origin. Unlabelled illustration

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