Claim: Down syndrome is an abnormality of chromosomes.

Chromosome 21 is one of the 23 pairs of chromosomes in humans . Chromosome 21 is the smallest human autosome , with 48 million nucleotides -LRB- the building material of DNA -RRB- representing about 1.5 percent of the total DNA in cells . People without Down 's syndrome have two copies of chromosome 21 , while those with three copies of chromosome 21 have Down syndrome , also called `` trisomy 21 '' .   Researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome . Chromosome 21 was the second human chromosome to be fully sequenced , after chromosome 22 .