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Tetrasomy 18p: Sixty-Second Summary

(Aliases: isochromosome 18p)

Key points on genotype

  • Nearly all individuals with Tetrasomy 18p have the same genotype.
  • Definitive diagnosis requires:A chromosome microarray to determine the precise region of net copy number change.
  • A karyotype to demonstrate that the copy number change is due to an isochromosome.
  • Almost all are de novo events as opposed to inherited from a parent.
  • There have been a few case reports of parents with mosaicism or with a chromosome rearrangement.
  • Parents may consider chromosome analysis to better define risks for future pregnancies.

Key Points on phenotype

  • Most individuals are not medically fragile.
  • Developmental delay is very common.
  • The average full scale IQ score  is 48, though there is a wide range of ability.
  • Behavioral concerns are common.
  • Life expectancy is believed to be near normal.
  • Congenital anomalies are possible. Specifically, individuals with Tetrasomy 18p have an increased likelihood of myelomeningocele, heart defects, hernias, palate abnormalities and orthopedic abnormalities.


  • Affected individuals do not appear to be at increased risk for adverse reactions to drugs or standard medical treatments.
  • Recommendations for specific evaluations  and treatments are in the following sections.


  • The Chromosome 18 Clinical Research Center is enrolling anyone with any chromosome 18 abnormality in our longitudinal study of all aspects of the conditions.
  • Parents may contact Annice Hill at or call (210) 567-5321.
  • Enrollment requires the diagnostic genetics report and any other informative medical records


  • Daniel Hale, MD, Medical Director of the Chromosome 18 Clinical Research Center can be reached through Annice Hill at or call (210) 567-5321.

Updated 2020