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FG syndrome
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

FG syndrome

FG syndrome is an X-linked genetic condition characterized by intellectual disability, behavioral differences, and specific physical features, occurring almost exclusively in males.

Overview

FG syndrome affects many parts of the body and occurs almost exclusively in males. The name "FG" represents the surname initials of the first family diagnosed with the disorder. While intellectual disability is common, affected individuals often possess strong socialization and daily living skills, though verbal communication and language skills tend to be weaker.

Symptoms

  • Intellectual and behavioral features: Intellectual disability (mild to severe), hyperactivity, short attention span, and a friendly, inquisitive nature.
  • Physical features: Weak muscle tone (hypotonia), broad thumbs, and wide big toes.
  • Facial appearance: Small, underdeveloped ears; a tall, prominent forehead; and down-slanting eyes (down-slanting palpebral fissures).
  • Brain and structural abnormalities: Abnormalities of the corpus callosum (the tissue connecting the two halves of the brain).
  • Gastrointestinal issues: Constipation and potential anal abnormalities, such as an obstruction of the anal opening (imperforate anus).
  • Other health concerns: Heart defects, seizures, undescended testes (cryptorchidism), and inguinal hernia.

Causes

The disorder is linked to changes in five regions of the X chromosome. Mutations in the MED12 gene are the most common cause. The MED12 gene provides instructions for a protein that helps regulate gene activity by turning genes on and off. This protein is essential for development both before and after birth. While these mutations alter the structure of the MED12 protein, the exact mechanism leading to the specific symptoms of FG syndrome remains under investigation.

Inheritance

FG syndrome is inherited in an X-linked recessive pattern. Because the genes associated with the disorder are located on the X chromosome, males—who have only one X chromosome—are affected if they inherit one altered copy. Females, who have two X chromosomes, typically require mutations in both copies to be affected, making the condition much rarer in females. Fathers cannot pass X-linked traits to their sons.

Frequently Asked Questions

Initial workup must include an echocardiogram to evaluate for structural heart defects, a brain MRI to assess the corpus callosum and ventricular anatomy, a renal ultrasound, an evaluation of the spine and anorectal anatomy, and comprehensive baseline audiology and ophthalmology exams. Periodic neurological monitoring is also required to identify emerging seizure activity.
FG syndrome is caused by mutations on the X chromosome (most commonly MED12). Males, having only one X chromosome, exhibit the full clinical spectrum. Females who carry one mutated copy are generally asymptomatic or display very subtle traits (such as minor facial features or obstinate constipation) due to X-inactivation. An unaffected female carrier has a 50% chance with each pregnancy of having an affected son and a 50% chance of having a carrier daughter.
Individuals with FG syndrome typically have an affectionate, social, and inquisitive personality, but they often struggle with attention-deficit/hyperactivity disorder (ADHD), low frustration tolerance, and sensory processing issues. Positive reinforcement, predictable daily schedules, sensory breaks, and tailored behavioral interventions that leverage their strong social drive are highly effective, while verbal tasks should be simplified to accommodate expressive language limitations.
Chronic constipation in FG syndrome stems from a combination of generalized hypotonia, structural anorectal malformations (such as anal stenosis or imperforate anus), and underlying dysmotility of the enteric nervous system. Management requires early evaluation by a pediatric gastroenterologist and pediatric surgeon, long-term maintenance on osmotic laxatives (like polyethylene glycol), aggressive hydration, high-fiber diets, and proactive disimpaction regimens to prevent bowel obstruction or megacolon.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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