The consequence of trisomy 21 in humans.
That is, there are three copies or, at least, three partial copies of chromosome 21. Recall that a syndrome is a constellation of signs and symptoms.
Of the autosomal aneuplodies, Down syndrom is the most common and this is due to the much higher potential for the survival among afflicted individuals. That likelihood of survival, in turn, reflects at least in part the smallness of chromosome 21, which is the smallest of human chromosomes, which implies that the expression of fewer genes will experience dosage problems.
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