Apparently all Alzheimer’s disease patients have some cells with three copies of chromosome 21 rather than the normal two. People born with Downs syndrome share in the trait, except that they are born with “trisomy 21” in all cells. In the case of Alzheimer’s the trisomy 21 is probably due to damage inflicted in the cell microtubules by the amyloid protein, disrupting chromosomal replication. It is also suspected of causing the diabetes, strokes and atherosclerosis that is seen in many Alzheimer’s patients. ScienceDaily has more details about the research.