The three chromosomal abnormalities that cause Down's syndrome are:

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Down's syndrome is caused by the presence of an extra copy of chromosome 21. This can result from trisomy 21, where there are three copies of chromosome 21 instead of the normal two, or from a translocation where a portion of chromosome 21 becomes attached to another chromosome. Mosaicism, where some cells have the extra chromosome 21 and some do not, is also a less common cause of Down's syndrome. Trisomy 23 and Fragile X are not associated with Down's syndrome.

The Three Chromosomal Abnormalities That Cause Down's Syndrome Are