Mosaic Down syndrome is a rare form of Down syndrome — a genetic condition that causes an extra chromosome to appear in a person’s DNA.

Human DNA typically contains 23 pairs of chromosomes, making 46 in total. These chromosomes contain the information that tells the cells how to grow.

Babies born with trisomy 21 Down syndrome, the most common form of the condition, have an extra copy of the 21st chromosome. This occurs due to a genetic feature in either the sperm or the egg of a parent.

As a result, the extra chromosome in the DNA gets copied into each of the cells of a developing baby with trisomy 21 Down syndrome.

Meanwhile, in mosaic Down syndrome, there is a mixture of cells — some cells contain the standard 23 pairs of chromosomes, and some contain the extra copy of chromosome 21.

Babies born with mosaic Down syndrome may have similar features and health issues as those born with trisomy 21 Down syndrome. Overall, though, they may have fewer of these characteristics.

Currently, very little research has compared the effects of mosaic Down syndrome with those of the more common form of the condition, but research is ongoing.

a child with Mosaic down syndrome looking at a ferris wheelShare on Pinterest
A child with mosaic Down syndrome may have some of the health issues of a person with trisomy 21 Down syndrome.

Doctors and researchers use the term “mosaicism” to describe a mix of cells in the body.

While people with the more common trisomy 21 Down syndrome have an extra chromosome in all of their cells, people with mosaic Down syndrome only have the extra chromosome in some cells.

Because of this, people with mosaic Down syndrome may have fewer characteristics of the condition than people with trisomy 21 Down syndrome.

Physical characteristics include:

  • almond-shaped eyes that tend to slat upward
  • a neck that is shorter than average
  • a flattened face, particularly near the bridge of the nose
  • small ears
  • white spots on the iris of the eye
  • poor muscle tone
  • loose joints
  • shorter height or slower growth
  • small hands and feet
  • small pinky fingers
  • a tongue that sticks out of the mouth

Down syndrome can also involve:

  • disorders of the eyes and trouble seeing
  • disorders of the ears and trouble hearing
  • a compromised immune system
  • neurological disabilities

Children born with Down syndrome may develop more slowly than average. In general, children with Down syndrome are slower to speak and may have average or below-average IQs.

A child with mosaic Down syndrome — who may have fewer characteristics of the condition than a child with trisomy 21 Down syndrome — may perform better on cognitive tests, for example.

However, characteristics of Down syndrome vary from person to person. Comparative research is still ongoing.

Mosaic Down syndrome occurs when an extra copy of the 21stchromosome is passed into the genes shortly after conception. How early on this occurs may help determine how many cells are affected, as only some of the body’s cells will carry this extra chromosome.

Researchers are still unsure of the specific factors that increase the likelihood of this genetic condition.

However, maternal age may play a role. The Centers for Disease Control and Prevention (CDC), for example, report that women who are 35 or older when they conceive are more likely to have pregnancies affected by Down syndrome.

Typically, a Down syndrome diagnosis involves testing the genetic material in the blood. A doctor may isolate 20–25 cells and test them for an extra copy of the 21st chromosome.

If all the cells have the extra copy, this indicates trisomy 21 Down syndrome. However, if some cells have a copy while others do not, this indicates mosaic Down syndrome.

If only some cells carry an extra copy of the 21st chromosome, the doctor may recommend other tests, involving skin or brain cells. These additional tests can help confirm the diagnosis.

Some diagnostic tests can determine whether a fetus has Down syndrome. They involve testing a sample of cells from the placenta or amniotic fluid.

Managing mosaic Down syndrome depends on the characteristics of the condition, and these can vary from person to person.

Physical therapy may help improve the quality of life and reduce any physical limitations. Meanwhile, language, speech, and occupational therapy may help enhance social skills.

Doctors work closely with the parents of children with mosaic Down syndrome to identify and treat any complications, such as heart and lung conditions, that may develop.

Managing characteristics of mosaic Down syndrome may greatly improve a person’s physical and mental capabilities and their overall quality of life.

When a person has mosaic Down syndrome, the range of possible complications varies widely. This makes it difficult to make broad predictions about their health or life expectancy.

Managing any complications and accompanying health issues can extend life expectancy.

Among people with Down syndrome in general, conditions affecting the heart and lungs account for about 75% of deaths, according to research published in the European Respiratory Review.

However, people with mosaic Down syndrome may experience less severe effects of complications than people with 21 trisomy Down syndrome, according to a review in the American Journal of Medical Genetics.

Overall, a population-based study in Genetics in Medicine found that people with mosaic Down syndrome appear to have better survival rates than people with trisomy 21 Down syndrome.

The study confirmed that younger people with more access to treatment for serious complications, such as congenital heart abnormalities, had higher life expectancies.

Each instance of mosaic Down syndrome is unique. It is important to discuss each person’s experience and situation with healthcare professionals.

Mosaic Down syndrome occurs when a person has a mixture of typical cells and cells with an extra chromosome.

The exact features of mosaic Down syndrome vary widely from person to person. Some people have few or no characteristics of the condition, while others have more, and their experience may be closer to that of someone with the more common trisomy 21 Down syndrome.

For everyone with Down syndrome, it is important to work closely with doctors to help identify and manage symptoms and complications. Doing so can improve overall health and the quality of life.