Karyotype showing the arrangement of chromosomes in a male with Down's syndrome (trisomy-21), the most common diagnosable cause of mental handicap. 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 male set differs from the female in the sex pair (bottom right); a female would have XX rather than XY. A male Down's karyotype is written: 47,XY,+21. The normal chromosome count is 46, 23 from maternal and 23 from paternal origin.