A century after Down syndrome was first described, scientists discovered that the root cause of this disorder is a condition called trisomy 21 ... chromosome, usually chromosome 14 or 15.

When the egg and sperm unite, all resulting cells carry three copies of chromosome 21. This type, known as complete trisomy 21, accounts for nearly all cases of Down syndrome. Mosaic Trisomy 21 ...

This can be done by analysing genotype-phenotype correlations in rare patients with partial trisomy 21. Thus, karyotypic studies have indicated that trisomy of the distal part alone of chromosome ...