Down Syndrome is caused by changes to the 21st chromosome that occur
during cell division in the early stages of pregnancy.
Three genetic variations can lead to Down syndrome. The 21st
chromosome does not divide but instead remains whole, causing an
'extra copy' of itself. (Non-disjunction) Or part of the 21st
chromosome can break away (Translocation) and join another
chromosome, creating repeated genetic material. Sometimes there is a
mix of cell lines, some of which have a normal set of chromosomes
and another set, which have the extra copy of chromosome 21. This
form of Down syndrome is called Mosaicism.
When the changes to the chromosome occur they affect DNA (our
genetic makeup) and our cells, this leads to various changes
throughout the bodily systems.
Although there are varying degrees of change, they usually result in
delayed development, (this means development is slower than what
might be considered 'normal' for the average child.) in cognitive,
pathological and physical aspects.
The change to the chromosomes may also bring out into the open any
'weaknesses' that might be present in cell lines and so children
with Down syndrome may be more susceptible to ailments that are
already within the general public, such as thyroid dysfunction,
diabetes and heart conditions as well as decreased hearing and
poorer vision.
The types of Down syndrome, as mentioned above, only tell us how the
genetic change occurred, in each case; the resulting condition can
range from mild to severe.
Nondisjunction. Approximately 92% of the time, the most
common form of Down syndrome is caused by the presence of an extra
chromosome 21 in all cells of the individual. In such cases, the
extra chromosome originates in the development of either the egg or
the sperm. Consequently, when the egg and sperm unite to form the
fertilized egg, three--rather than two--chromosomes 21 are present.
As the embryo develops, the extra chromosome is repeated in every
cell. This condition, in which three copies of chromosome 21 are
present in all cells of the individual, is now often referred to
medically as Trisomy 21
Translocation. Approximately 3-4% of individuals with Down
syndrome have cells containing 46 chromosomes and not 47, which is
the most typical representation of the extra copy of chromosome 21.
These individuals still have
the features associated with Down syndrome because the material from
one chromosome 21 gets stuck or translocated (moved) onto another
chromosome. As there is still much genetic material from
chromosome21, it still results in the traits associated with Down
syndrome, although the normal amount of Chromosomes. This Variation
of Down syndrome is referred to medically as Translocation Trisomy
21
Mosaicism. In approximately 2-4% of cases, Down syndrome is
due to Mosaic Trisomy 21. In this instance, the extra chromosome 21
is present in some, but not all, cells of the individual. For
example, the fertilized egg may have the right number of
chromosomes, but, due to an error in chromosome division early in
embryonic development, some cells acquire an extra chromosome 21.
Thus, an individual with Down syndrome due to mosaic trisomy 21 will
typically have 46 chromosomes in some cells, but will have 47
chromosomes (including an extra chromosome 21) in others. In this
situation, the range of the physical problems may be less depending
on the proportion of cells that carry the additional chromosome 21.
Health Notes
Due to the more than usual amount of health problems caused by Down
syndrome, many children are often born with heart abnormalities,
some may clear up on their own in time and some may require surgery
urgently or at a later date.
To this end it is important to make sure all follow-up cardiac
appointments are attended.
Regular appointments should also be made for E.N.T and vision.
A thyroid test should be done after the birth and thereafter yearly.
If your child sweats profusely or has difficulty maintaining body
temperature it is always sensible to check with your Doctor.
Down Syndrome is caused by changes to the 21st chromosome that occur
during cell division in the early stages of pregnancy. 
