False-colour karyotype depicting the arrangement of homologous pairs of chromosomes in a female with Down's syndrome (trisomy-21). The syndrome is caused by a chromosomal anomaly: the 21st set having three rather than the normal two chromosomes. The defect results in mental retardation & physical disability. Chromosomes are arranged in numbered pairs according to a standard classification. The male set of chromosomes differs only in the sex chromosomes (far left), which would have shown Y (XY) rather than X (XX). The nucleus of each human cell contains 46 chromosomes in total: 23 from a maternal & 23 from a paternal origin.