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What is Mosaic Down Syndrome?

Tricia Christensen
Updated Mar 03, 2024
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Down syndrome or Trisomy 21 refers to the presence of three instead of two 21st chromosomes in each of the body’s cells. This results in cognitive impairment, slightly different appearance, and risk for certain health issues or birth defects. Mosaicism is a term used to describe a condition where people have different kinds of cells in the body. Mosaic Down syndrome can thus be understood as a condition where people have some cells with the Trisomy 21 arrangement, and some cells that only posses two #21 chromosomes.

Mosaic Down Syndrome remains a rare variant of Trisomy 21, and it’s usually tested for in a blood sample. Doctors look for evidence of any cells with the normal arrangement to show whether this condition is present. Number of normal cells is expressed in a percentage value based on the Trisomy cells. A measurement of 80 out of 100 trisomy cells would be called 80% mosaicism. Percentages may vary depending on the part of the body tested.

From a logical standpoint, it would make sense to think that lower percentages of mosaicism would always correspond to less expression of Down syndrome. This isn’t necessarily the case, and it doesn’t seem that percentage will always predict the degree to which people are affected. In general it can be said that many people with Mosaic Down Syndrome have fewer symptoms, but this isn’t always the case.

Though a medically interesting variant, this type of Down syndrome is sometimes just as likely as total Trisomy 21 to have the following symptoms:

  • Impaired cognition, which may be slight to major
  • Characteristic appearance of smaller hands, short neck, tilted eyes, small mouth, flattened nose, and shorter stature
  • Great risk of congenital heart defects (about 50% of kids with Down syndrome)
  • Higher risk for leukemia and thyroid dysfunction
  • More likelihood of vision and hearing problems

Some huge differences exist between Mosaic Down syndrome and the standard syndrome expression. First, most Down cases are diagnosed prior to birth or at birth. Mosaicism may not be diagnosed until children are between 1 and 4. It’s also the case that Mosaic Down syndrome is much rarer than standard Trisomy 21. An estimated 3% of Downs’s cases show mosaicism.

The child with Trisomy 21 or Mosaic Down Syndrome does have many difficult things to overcome. These children may need more medical intervention than a child without a third #21 chromosome and they also benefit from early learning opportunities. Despite the deficits accompanying this condition, many of these kids can do well, be mainstreamed in school environments to a certain point, and hold jobs as adults while they live independently or semi-independently in group living settings.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By ZipLine — On Dec 29, 2014

I just learned that the risk of chromosomal abnormalities, like the one causing Mosaic Down syndrome depends on the mother's age. The older the mother is, the greater the risk is.

I didn't know this earlier. I'm nearing forty though and this is now one of the risks on my mind as I prepare to have a baby in the near future.

Does anyone here have a child with Mosaic Down Syndrome? Has it been difficult for you especially considering that this chromosomal abnormality doesn't become apparent well after the child is born?

By ddljohn — On Dec 28, 2014

@turquoise-- Like the article said, it's not possible to generalize about this. A child with Mosaic Down Syndrome may have the same level and variety of issues as a child with Down Syndrome or he may have less issues. It's basically impossible to predict and parents will find out as the child become a little older.

I think that one of the reasons for this variation may be because the presence of the third 21st chromosomes may vary from body part to body part. So for example, if many of the third 21st chromosomes are present in the brain, that child may experience more cognitive issues. Whereas if more of the third 21st chromosomes are present in the muscles, the child may have more issues with physical movement.

By turquoise — On Dec 27, 2014

I too assumed that fewer presence of three 21st chromosomes in cells would mean that individuals with Mosaic Down Syndrome would have less symptoms. It's kind of interesting why that isn't the case. I guess a child with Mosaic Down Syndrome needs the help and attention that children with Down Syndrome require.

By Lostnfound — On Dec 22, 2014

For some reason, I thought Mosaic Down Syndrome meant the child usually had a lot more health and delayed development issues than other Down children. The one child I knew who had the mosaic kind certainly did. He and his parents went to church with us and he was forever having all kinds of health problems, as well as not being verbal or able to walk very well. I always attributed that to the mosaic variation, but apparently, that's not the case.

Seems like that young man was in his teens when he died, but he may have been in his early 20s. He was very young, at any rate. It was very sad.

By Grivusangel — On Dec 22, 2014

My co-worker had a son who had Mosaic Down Syndrome. He was diagnosed with DS when he was about three months old, and with mosaicism when he was a little over a year old. He had a heart murmur, which is very, very common with Down babies, but his muscle tone in his upper body was not a good as it is with most Down babies, and theirs isn't usually good, to start with.

He got around by scooting on his bottom and using one leg to propel himself. His lower body strength was actually very good. He also had problems with his ears, which necessitated having tubes put in when he was about 18 months old.

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia...
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