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Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by mild-to-moderate intellectual disability. [1] The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled.
While initially described as affecting male carriers, female carriers of the FMR1 gene mutation have also been found to develop FXTAS. However, due to X-inactivation, female carriers are much less likely to develop dementia or classic ataxia and tremor, instead demonstrating symptoms such as fibromyalgia, thyroid disease, hypertension, and ...
Fragile X-associated primary ovarian insufficiency (FXPOI) is the most common genetic cause of premature ovarian failure in women with a normal karyotype 46,XX. [1] The expansion of a CGG repeat in the 5' untranslated region of the FMR1 gene from the normal range of 5-45 repeats to the premutation range of 55-199 CGGs leads to risk of FXPOI for ovary-bearing individuals. [2]
A karyotype (to evaluate for Turner's Syndrome) and a Fragile-X premutation carrier analysis is also recommended, with additional genetic testing possibly being warranted based on family history of amenorrhea or early menopause or signs and symptoms of a genetic disorder. [10]
The zygotes merge early enough that there is no risk of them developing into conjoined twins. [14] Variations of this mechanism include fertilization of an ovum and its first or second polar body by two sperm. [15] 46,XX/46,XY can also be explained by a mosaic-based mechanism. A single zygote is formed from the fertilization of a normal X ovum.
The first trinucleotide repeat disease to be identified was fragile X syndrome, which has since been mapped to the long arm of the X chromosome. Patients carry from 230 to 4000 CGG repeats in the gene that causes fragile X syndrome, while unaffected individuals have up to 50 repeats and carriers of the disease have 60 to 230 repeats.
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Silencing of the FMR1 gene in Fragile X syndrome. FMR1 co-localizes with a rare fragile site, visible here as a gap on the long arms of the X chromosome.. A chromosomal fragile site is a specific heritable point on a chromosome that tends to form a gap or constriction and may tend to break [1] when the cell is exposed to partial replication stress. [2]