BIOPHOTO ASSOCIATES / SCIENCE PHOTO LIBRARY BIOPHOTO ASSOCIATES / SCIENCE PHOTO LIBRARY
Down's Syndrome karyotype. Coloured Light Micro- graph of the karyotype (full set of chromosomes) of a Down's syndrome female. Here, the 46 chromosomes are arranged in their 23 homologous pairs. Down's syndrome (trisomy-21) is caused by a chromosomal anomaly: the 21st set has three rather than the normal two chromosomes. The defect results in mental retardation and physical dis- ability. The male set of chromosomes differs only in the sex chromosomes (lower right), which would have shown XY rather than XX for female. The nucleus of each human cell contains 46 chromo- somes, structures containing DNA which determines the characteristics of the individual.
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