Down syndrome is usually caused by trisomy 21, or having three of chromosome number 21 instead of the usual two copies. In most cases this occurs because of nondisjunction in the egg cell, so that it carries two copies of chromosome 21. There are, however, a small number (1-4%) of Down syndrome individuals who have what is known as mosaicism, where the egg and sperm are both normal, and the nondisjunction occurs during the development of the embryo. Individuals with mosaic Down syndrome have a mixture of trisomy and normal cells, and often have less severe Down symptoms.
Generally Down syndrome individuals are infertile, and they also frequently suffer from heart abnormalities, so a Down woman would be at high risk during a pregnancy. However there have been some (around 30 are known) who have successfully carried to term; according to the literature, about a third of the babies have been normal.
Trisomy cells undergoing meiosis could result in some normal gametes, since the three chromosomes cannot be split up evenly. Additionally, a Down syndrome individual who has the mosaic condition could have sufficient normal cells in the reproductive tract to produce normal egg or sperm cells.