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what causes fragile x syndrome

Fragile X syndrome is caused by a specific change (mutation) in a single gene on the X chromosome called the FMR1 gene.

Quick Scoop: Core Cause

  • The FMR1 gene normally contains a repeated three-letter DNA sequence, “CGG,” at one spot.
  • In fragile X syndrome, this CGG repeat expands far beyond normal and becomes very long (over about 200 repeats, called a “full mutation”).
  • This extra-long repeat leads to heavy “methylation” (a chemical silencing tag) on the gene, which switches the gene off so it can’t work properly.
  • When FMR1 is silenced, the body makes little or none of the FMRP protein, which is needed for normal brain development and connections between nerve cells.
  • The result is the learning, behavior, and physical features associated with fragile X syndrome.

How the Gene Change Works

Think of the FMR1 gene as a factory blueprint for a key brain protein, FMRP.

  1. In most people:
    • The CGG section repeats a normal number of times.
    • The blueprint is readable, so the factory produces enough FMRP.
  2. In fragile X syndrome:
    • The CGG section expands into a huge block of repeats (full mutation).
    • Chemical tags (methyl groups) are added, “blacking out” that blueprint.
    • The factory cannot make enough FMRP (or any at all).

Without FMRP, the brain’s wiring—especially the connections (synapses) between neurons—doesn’t develop or adjust normally, which underlies the intellectual disability and behavioral symptoms.

Is It Inherited?

  • Fragile X is an inherited condition with X-linked dominant inheritance, meaning the mutation is on the X chromosome and can be passed down in families.
  • Many carriers have a “premutation” (an intermediate number of repeats) that can expand to a full mutation when passed to children, especially through the egg cells of a carrier mother.
  • Both males and females can be affected, but males (with only one X chromosome) often have more severe symptoms.

What Does Not Cause Fragile X

  • It is not caused by parenting style, infections during pregnancy, vaccines, or environmental toxins in everyday life.
  • The root cause is this specific FMR1 gene mutation and the resulting lack of FMRP, even though environment and support can influence how a child functions day to day.

TL;DR: Fragile X syndrome happens when a CGG repeat in the FMR1 gene on the X chromosome becomes massively expanded, chemically silences the gene, and prevents the brain from making enough FMRP, a protein crucial for normal brain connections.

Information gathered from public forums or data available on the internet and portrayed here.