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XXX (triple X) Syndrome
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What is a chromosome disorder and what is its significance?
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Chromosomes are the structures
in which our hereditary material (DNA) is packed.
A healthy person has 46 chromosomes (23 pairs). One chromosome of each pair (i.e.
a total of 23 chromosomes) is donated by each parent to the fetus. Twenty-two pairs
are called autosomes, and the 23rd pair consists of the sex chromosomes (X and Y).
Females have two X chromosomes and males have one X and one Y. The chromosome pairs
are arranged and numbered by size and shape, and each has different genes. Because
there are two copies of each chromosome, there is effectively a duplicate gene system
(one maternal and the other paternal), each identical in sequence to the other.
A change in the number or structure of the chromosomes in pairs 1 - 22, causing
an addition or deletion of chromosomal information in these pairs compared with
the normal state, constitutes a significant medical problem. In the great majority
of cases, this change is expressed as severe structural or functional defects in
the tissues and/or mental retardation. For example, an extra number
21 chromosome, resulting in three instead of the normal two so that there are 47
chromosomes altogether, causes Down
syndrome. An extra number 18 chromosome, which is larger and therefore contains
more genes, manifests as a more severe disorder called Trisomy 18. In this syndrome, there are severe defects,
and it usually ends in death soon after birth or even in the uterus. This is also
the case with a deletion of the whole or a part, however small, of chromosome 18.
Conversely, the clinical manifestation of a change in the number of sex chromosomes
(X or Y) or their structure, as is the case in XXX syndrome, is usually less severe.
See under "chromosomes".
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What is XXX syndrome?
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In this syndrome there is an extra X chromosome in addition to the two that are
normally present in females, giving a total of 47 chromosomes altogether.
About 70% of these female fetuses survive through to term (birth), and the frequency
is 1 in 1,000 female births.
The only external characteristic is tall stature relative to other family members
- there are no other physical changes.
Developmentally, there are usually no problems in the first year of life. Later,
however, there is delay in speech development in about 50% of cases. In the school
years, speech and language problems, as well as immature behavior, are frequently
encountered.
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Some of these girls have perfectly normal intelligence and go undiagnosed, but some
may have mental retardation at varying levels of severity through to severe retardation.
Their average intelligence quotient (IQ) is about 15 - 20 points lower than their
healthy siblings. The risk of mental retardation of varying levels is therefore
high (5 - 10 times more than the regular risk). About 50% have learning problems
that require special settings in some cases, and in addition, psychological and
psychiatric problems and problems of social adjustment have been described.
A small number also have defects in the development of the uterus and its appendages.
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What is the risk of recurrence?
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XXX syndrome is not hereditary and does not result from a problem in the parents.
A couple that has had a fetus with XXX syndrome in a previous pregnancy has only
a 1% risk for recurrence of chromosomal problems in another pregnancy. Each such
couple may undergo amniocentesis in each future pregnancy, and, as stated, in 99%
of cases the problem does not recur. More distant relatives are not at significantly
increased risk.
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