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The three forms of Down syndrome are called simple trisomy,
translocation and mosaicism.

Simple Trisomy
About 95 percent of all people with Down syndrome have this particular form.
Parents will have normal chromosomes, but their baby has three number 21parentlink3
chromosomes, rather than the usual pair. This is an accidental occurrence that
happens in the division of the cell. Although the birth of a child with Down syndrome
is slightly more common among older parents, it can occur at any age.

As the name suggests, this means movement to another location, where a segment
of number 21 chromosome actually breaks off and attaches to another chromosome.
This is observed in about 4 percent of babies with Down syndrome and can take
several forms. In all cases, the baby has a normal number of chromosomes, but has
extra chromosomal material i.e. as well as the usual two number 21 chromosomes, the
baby has an extra portion of number 21 chromosome attached to a normal
chromosome. In about one third of these children either parent may carry a
translocation but show no signs or symptoms. The mother’s age is not an important
factor in this type of Down syndrome.

In one percent of people with Down syndrome, body cells have a mosaic pattern.
This means that there is an extra whole number 21 chromosome in only some of the
body cells, the rest of the cells are normal/or have 46 chromosomes. As a result of this
mixture, babies with a mosaic cell pattern may have less prominent physical features
of the syndrome.

While statistics vary in other countries, in Ireland it is
estimated that about one baby in every 600 born has Down
syndrome. As yet, it is not known what causes Down syndrome.