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Turner Syndrome |
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What is a chromosome disorder and what is its significance? |
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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