Description
Labels Fragile X
Fragile X syndrome is a genetic disorder caused by a mutation in the FMR1
gene on the X chromosome. The disorder was first described in 1943 by two
psychiatrists named Martin and Bell, who determined that the syndrome is
sex-linked.The specific gene involved, however, was not identified until 1991.
Themutation consists of the expansion of a section of the gene that ordinarily
consists of six to fifty-five repeats of a specific trinucleotide (tripletnucleotide;
nucleotides are one of the building blocks of DNA). In some
people, this portion of the FMR1 gene contains more than fifty-five repeats
of the triplet. People with sixty to 200 repeats have what is called a premutation;
they do not have the typical symptoms of fragile X syndrome but they
can carry the defective gene and pass it on to their children. The repeated
portion of the FMR1 gene is likely to have more repetitions added when it
is passed from a woman with the premutation to her children. When the
number of repetitions grows to about 230, the gene is switched off and does
not produce the protein that it normally makes. This gene change is then
considered a full mutation. Some people with fragile X syndrome have as
many as 1,000 repetitions of the crucial triplet in the FMR1 gene.
Because the X chromosome is a sex-linked chromosome, fragile X
syndrome affects boys more severely than girls. Girls have two X chromosomes,
whereas boys have one X chromosome and one Y chromosome. A
boy who inherits a full mutation of the FMR1 gene will develop fragile X
syndrome because his only X chromosome contains the mutated gene.
A girl who inherits a full mutation may not be as severely affected
because each cell of her body needs to make use of only one of its
two X chromosomes; it can inactivate the other X chromosome.
People with a fragile X premutation are usually of normal intelligence
but may suffer from other health problems. About 20 percent
of women with the premutation stop having menstrual periods unusually
early, often by age forty. Men and some women with the premutation
are at increased risk of developing a movement disorder accompanied
by memory loss, tremor, and loss of sensation in the lower legs.
gene on the X chromosome. The disorder was first described in 1943 by two
psychiatrists named Martin and Bell, who determined that the syndrome is
sex-linked.The specific gene involved, however, was not identified until 1991.
Themutation consists of the expansion of a section of the gene that ordinarily
consists of six to fifty-five repeats of a specific trinucleotide (tripletnucleotide;
nucleotides are one of the building blocks of DNA). In some
people, this portion of the FMR1 gene contains more than fifty-five repeats
of the triplet. People with sixty to 200 repeats have what is called a premutation;
they do not have the typical symptoms of fragile X syndrome but they
can carry the defective gene and pass it on to their children. The repeated
portion of the FMR1 gene is likely to have more repetitions added when it
is passed from a woman with the premutation to her children. When the
number of repetitions grows to about 230, the gene is switched off and does
not produce the protein that it normally makes. This gene change is then
considered a full mutation. Some people with fragile X syndrome have as
many as 1,000 repetitions of the crucial triplet in the FMR1 gene.
Because the X chromosome is a sex-linked chromosome, fragile X
syndrome affects boys more severely than girls. Girls have two X chromosomes,
whereas boys have one X chromosome and one Y chromosome. A
boy who inherits a full mutation of the FMR1 gene will develop fragile X
syndrome because his only X chromosome contains the mutated gene.
A girl who inherits a full mutation may not be as severely affected
because each cell of her body needs to make use of only one of its
two X chromosomes; it can inactivate the other X chromosome.
People with a fragile X premutation are usually of normal intelligence
but may suffer from other health problems. About 20 percent
of women with the premutation stop having menstrual periods unusually
early, often by age forty. Men and some women with the premutation
are at increased risk of developing a movement disorder accompanied
by memory loss, tremor, and loss of sensation in the lower legs.
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