This is by far the most common of a range of conditions called trisomies, in which one pair of chromosomes has an extra chromosome, making three. In Down syndrome, there are three number 21 chromosomes. Affected infants characteristically have small features, a tongue that protrudes, and slanting eyes with folds of skin at their inner corners. They tend to be rather floppy infants and have short, wide hands. They may also suffer from congenital heart disease.

Down syndrome babies usually mentally handicapped, though the degree of handicap varies widely; many are near normal. They are usually affectionate and happy children. With careful attention and early education, they often do very well, and some manage to live independently as adults.