How is Down syndrome characterized?

Updated: Apr 30, 2018
  • Author: Gratias Tom Mundakel, MBBS, DCH; Chief Editor: Maria Descartes, MD  more...
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Down syndrome is by far the most common and best known chromosomal disorder in humans and the most common cause of intellectual disability. It is primarily caused by trisomy of chromosome 21 (see the image below), which gives rise to multiple systemic complications as part of the syndrome. However, not all defects occur in each patient; there is a wide range of phenotypic variation. [1]

G-banded karyotype showing trisomy 21 (47,XY,+21). G-banded karyotype showing trisomy 21 (47,XY,+21).

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