If a person with Down syndrome marries a person without Down syndrome , is it possible for their child not to have Down syndrome? Please explain.

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 href="http://www.mosaicdownsyndrome.com/faqs.htm">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.