AP Biology
Fragile X syndrome is the most common inherited cause of autism and intellectual disabilities. "It affects 1 in 4000 boys and 1 in 6000 girls of all races, worldwide and is a recessive inherited disease. Fragile X syndrome is a very rare disease and affects 1 in 5000 people." (FRAXA). It occurs when a single gene, FMR1, on the long arm of the X chromosome, shuts down and fails to produce a protein, FMRP, which is vital for normal brain development.
"About 1 in 259 women carry fragile X and could pass it to their children. About 1 in 800 men carry Fragile X; their daughters will also be carriers." (FRAXA). It is one of the most common genetic diseases in humans and is the #1 inherited disease that causes intellectual disabilities and is the most common cause of autism. Most people with Fragile X do not even know it and go undiagnosed with another form of autism.
The FMR1 gene is located on the long arm of the X chromosome. At the start of this gene of DNA, there is a part of it that can vary in length from person to person. In some people, this part of the DNA is somewhat longer; this gene change is a permutation. Although a person who carries the premutation does not typically have symptoms of Fragile X, can, the stretch of DNA is more likely to lengthen when it is passed from a woman to her children. When the stretch of DNA expands beyond a certain length, the gene is switched off and does not produce the protein FMRP anymore. A male who inherits a full mutation exhibits Fragile X syndrome because his only X chromosome contains the mutated gene. A female may not be as severely affected because each cell of her body needs to use only one of its two X chromosomes and randomly inactivates the other. "In women, the FMR1 gene premutation on the X chromosome can expand to more than 200 CGG repeats in cells that develop into eggs. This means that women with premutation have an increased risk of having a child with fragile X syndrome.
By contrast, the premutation in men does not expand to m...
