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Turner 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 Turner syndrome, is usually less
severe. See under Chromosomes.
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What is Turner syndrome?
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This syndrome is caused by the absence of one X chromosome in females, so that these
girls have only one X chromosome and a total of 45 chromosomes altogether.
This is monosomy X. Some cases occur when the monosomy is present in only some of
the cells, while others cells are normal - this condition is known as mosaicism.
The syndrome is reported to have a frequency of 1:1500 to 1:2500 live female births.
There is a wide spectrum of clinical signs of varying degrees of severity, the major
ones of which are short stature and ovaries that fail to develop, resulting in infertility
and lack of secondary sexual characteristics in the absence of hormonal therapy.
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A significant number of these girls have a characteristic build with a wide, webbed
neck.
Some also have heart defects such as aortic stenosis (narrowing of the aortic valve).
There is marked developmental variation. Some girls with this syndrome have normal
intelligence, but some have a cognitive deficit compared to healthy siblings, and
some need a supportive study framework and even possibly special education. In most
cases, however, there is no significant mental retardation.
significant percentage of fetuses with this syndrome abort spontaneously in the
early stages of pregnancy. In most of these cases there are accompanying defects,
such as heart defects.
The presence of the syndrome should be suspected if a webbed neck with a significant
cystic hygroma or diffuse edema is diagnosed, or when a heart
defect is detected by
ultrasound examination in the first trimester of pregnancy.
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Turner Syndrome
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