Down's syndrome. Coloured karyotype of the 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 (from top left to right in rows from top to bottom) 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.