21 Chromosome is the smallest human chromosome. It represents about 1.5% of the total DNA in cells. People normally have two copies of this chromosome as all other non-sex chromosomes. Chromosome twenty one spans about 46.9 million base pairs. Base Pairs are two molecules (nucleotides) on opposite DNA strands that are connected. Adenine (A) forms a base pair with thymine (T), as does guanine (G) with cytosine (C) in DNA. Base pairs is how DNA is measured. There are about 352 genes in this chromosome.

In Down syndrome, 95% of all cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these cases, approximately 90% of the abnormal cells are the eggs. The cause of the non-disjunction error isn't known, but there is definitely connection with maternal age. Research is currently aimed at trying to determine the cause and timing of the non-disjunction event.

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