Mosaic Down Syndrome

There are 3 types of Down syndrome disorders with mosaic being the rarest. 90 to 95% of people with Down syndrome have trisomy for chromosome 21. According to researchers, 2-4% of people with Down syndrome have mosaicism.

What is Down Syndrome?

Down Syndrome  is a chromosomal disorder caused by an extra cell division that results in an extra 21st chromosome. This causes developmental delays both intellectually and physically. It is the most commonly occurring chromosomal condition and is estimated that 1 in every 700 babies in the United States are born with Down syndrome.

Mosaicism

Mosaicism or Mosaic Down syndrome develops when there is a mixture of cells meaning some cells contain the extra copy of chromosome 21 while other cells are typical chromosomes.

Prevalence
  • It is estimated that 2 or 3 children diagnosed as having Down syndrome carry the mosaic form.
  • Mosaicism for trisomy 21 was first reported in 1961 by CM Clark, JH Edwards and V. Smallpeice who describe an 11-month old female with good muscle tone, no congenital heart defects, meeting developmental miles yet with eyes similar to babies born with Down syndrome.
  • By 1964, 24 cases of mosaicism were now on record.
  • Little is still known on the similarities and differences between trisomy 21 and mosaic trisomy.
Signs and Symptoms

People with Mosaic Down symptoms often have the same health issues as individuals with trisomy but less severe. This includes:

  • sleep apnea
  • heart issues
  • visual impairments
  • immune disorders
Physical Characteristics

People with Mosaicism vary in physical appearance depending on the number of trisomy cells.

People with Mosaic Down syndrome typically have a higher IQ and are more likely to hold a fulltime job compared to people with trisomy 21 Down syndrome.

Screening Test

Screening test are usually done as routine test during pregnancy health visits.

Updated 5/20/21

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