Mosaic Down Syndrome Fast Facts

Down syndrome is a genetic disorder that typically causes developmental delays and disabilities. It can also cause physical complications.

Down syndrome occurs in about 1 in every 700 babies.  Approximately 6,000 babies are born with Down syndrome in the United States each year.

Mosaic Down syndrome is a rare form of Down syndrome that usually causes similar symptoms.

Mosaic Down Syndrome accounts for about 1-2% of all cases of Down Syndrome.

Down syndrome is the most common genetic chromosomal disorder, and it is the leading cause of learning disabilities in children.

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Mosaic Down Syndrome accounts for about 1-2% of all cases of Down Syndrome.

What is Mosaic Down Syndrome?

Down syndrome is a genetic chromosomal disorder that causes cognitive and developmental delays and disabilities.  People with Down syndrome also often have physical complications associated with the condition.  Down syndrome occurs when a person’s cells contain extra genetic material that results from abnormal cell division before birth.

Mosaic Down syndrome is the rarest type of this disorder.  Its symptoms are typically similar to those of the other types but may be milder in some cases.

Aside from its developmental and health effects, Down syndrome causes distinctive physical features, including:

  • Flattened face
  • Upward slanting eyes
  • Small head
  • Short neck
  • Short stature
  • Short fingers and broad palms with a single crease in the skin
  • White flecks in the iris of the eyes
  • Unusual flexibility
  • Decreased muscle tone

Developmental Symptoms of Down Syndrome

Children with Down syndrome typically grow more slowly than other children, so their physical stature begins to lag through childhood.

The severity of cognitive and intellectual impairment varies significantly from individual to individual.  Some people have only mild impairment, while others are moderately affected.  Language development is typically delayed, and people with Down syndrome often have long- and short-term memory problems.

Potential Physical Complications of Down Syndrome

Down syndrome comes with an increased risk of physical complications, some of them severe enough to be life-threatening.

Common physical problems associated with Down syndrome include:

    • Heart defects.  Heart defects are present at birth in about half of babies born with Down syndrome.  These defects may require surgery.
  • Abnormalities in the digestive system
  • Abnormal development of the spine
  • Immune-related disorders
  • Dementia

Despite the potential for physical complications, life expectancy for people with Down syndrome continues to increase.  In 1960, the average life expectancy for an individual with the disorder was about ten years.  By 2007, the average life expectancy had risen to 47 years.  In addition, one study has suggested that people with mosaic DS may have higher life expectancies than those with other types.

What Causes Mosaic Down Syndrome?

The cells of people with Down syndrome have an extra copy (or an extra part of a copy) of a specific chromosome.  A chromosome contains the genes that control how our cells develop and behave.  A typical cell contains 23 pairs of chromosomes; one chromosome in each pair is inherited from the mother, and the other is inherited from the father. In Down syndrome, abnormal cell division before birth results in an extra copy of the chromosome.  The additional genetic material causes cells to develop and behave abnormally.

In mosaic Down syndrome, some of the person’s cells have an extra copy of chromosome 21, but others have the usual two copies. As a result, the effects of the disorder may be milder in this type, but that is not always the case.  Mosaic Down syndrome is the rarest type, accounting for only about 2% of cases.

Other Types of Down Syndrome

Three different types of Down syndrome are characterized by how abnormal chromosomal development happens in the cells.  In addition to mosaic Down syndrome, the other two types include:

  • Trisomy 21.  This type occurs when there is a separate extra copy of chromosome 21.  This is the most common type of the disorder, affecting about 95% of people with Down syndrome.
  • Translocation Down syndrome.  This type happens when an extra copy or extra fragment of chromosome 21 is attached to a different chromosome rather than a distinct extra copy.  This is a rare form, affecting only about 3% of people with Down syndrome.

Is Mosaic Down Syndrome Hereditary?

Down syndrome is a genetic disorder, but it is not usually passed on from parents to their children.  The abnormal cell division that results in the syndrome typically occurs spontaneously, not because of any specific genetic trigger in the cells.  This is true of mosaic Down syndrome as well as other types of the disorder.

Translocation Down syndrome, however, may be passed from parent to child.  In this case, the parent may possess some dislocated genetic material without having symptoms of the disorder themselves.  When this dislocated material is passed on, it can cause Down syndrome in the child.  However, not all translocation Down syndrome cases are inherited, and this type of the disorder is considered rare.

How Is Mosaic Down Syndrome Detected?

Down syndrome is often detected before birth.  In cases where it is not diagnosed before birth, the syndrome’s distinctive physical features are usually evident at birth.  It is unusual for the disorder to escape notice, even in its earliest stages.

Early intervention is essential, however.  Prompt treatment of potentially life-threatening physical complications is vital, and preventive measures can help decrease the risk of further complications in the future.  In addition, dealing effectively with the disorder and its effects can help support children as they cope with its challenges early in life.

How Is Mosaic Down Syndrome Diagnosed?

Detection of Down syndrome during pregnancy can be approached in two different ways.  Screening tests don’t directly detect Down syndrome, but they look for indications that the risk of Down syndrome is elevated.  Diagnostic tests look directly for the genetic signs of Down syndrome.  These tests are more definitive than screening tests, but they carry a risk to the mother and baby.  Therefore, they may not be used unless the pregnancy is considered high-risk.

Screening Tests

  • Blood tests.  These tests examine the mother’s blood for levels of hormones and proteins that could suggest Down syndrome.  These tests are routinely performed during all pregnancies.
  • Ultrasound.  An ultrasound imaging may reveal abnormalities in the baby’s neck that are characteristic of Down syndrome or other genetic disorders.

Diagnostic Tests

  • Chorionic villus sampling (CVS).  This test extracts cells from the placenta and examines them for indications of Down syndrome.  This test is usually performed during the first trimester of the pregnancy, and it carries a low risk of causing a miscarriage.
  • Amniocentesis.  This test examines a sample of the fluid in the amniotic sac surrounding the baby.  This test is usually performed in the second trimester, and it also carries a low risk of miscarriage.
  • Percutaneous umbilical blood sampling (PUBS).  This test examines blood from the umbilical cord for indications of Down syndrome.  It is less commonly used but may be ordered when other diagnostic tests are inconclusive.  Like other diagnostic tests, it carries a small risk of miscarriage and other complications.

Post-Birth Genetic Testing

Although a visual examination is typically enough to detect Down syndrome, sometimes physical features resembling those of Down syndrome are present in babies without the disorder.  Because of this, doctors will usually order a genetic blood test to confirm the diagnosis.

During this procedure, doctors test a small number of the baby’s cells (typically 20) to look for the presence of extra copies of chromosome 21.  When some, but not all, of the cells contain additional copies of the chromosome, the doctor can diagnose mosaic Down syndrome.  The proportion of normal cells to those with an extra chromosome is expressed as a percentage.  However, a relatively high percentage of normal cells does not necessarily mean that the disorder will be mild.


How Is Mosaic Down Syndrome Treated?

There is no cure for Down syndrome, and no treatment will reverse its effects.  However, treatments for health complications can prevent them from becoming severe or life-threatening.

Depending on each individual’s specific challenges, various therapies can help a person with Down syndrome live a full, productive, and happy life.

How Does Mosaic Down Syndrome Progress?

Mosaic Down syndrome uniquely affects each individual, and each person will face different complications and risks.  The disorder can cause a wide array of physical and health problems.  The challenges of the disorder’s developmental and cognitive effects can also put a person with Down syndrome at increased risk of still more physical and mental health complications.

Common complications of Down syndrome include:

  • Heart defects
  • Gastrointestinal defects
  • Vision and hearing impairment
  • Infections
  • Leukemia
  • Hypothyroidism
  • Spinal malformation
  • Celiac disease
  • Motor development delays
  • Epilepsy
  • Sleep disorders
  • Dental problems
  • Depression and anxiety
  • Attention-deficit/hyperactivity disorder
  • Dementia and Alzheimer’s disease

How Is Mosaic Down Syndrome Prevented?

The cause of the abnormal cell division that leads to Down syndrome is unknown.  This makes it challenging to find ways to prevent the disorder.  However, some factors increase the risk of Down syndrome.

  • Age of the mother.  The eggs of women over the age of 35 are at a greater risk of abnormal division, so the risk of Down syndrome is more significant when a woman becomes pregnant after that age.
  • Having a previous child with Down syndrome.  The risk of having a child with Down syndrome is greater for parents who have already had a child with the disorder.
  • Carrying genetic translocation.  Parents who carry translocated genetic material from chromosome 21 have a greater risk of having a child with Down syndrome.

A genetic counselor can help you assess the risk if any of these risk factors apply to you.

Mosaic Down Syndrome Caregiver Tips

As a caregiver for someone with mosaic Down syndrome, especially if that person is your child, you’re likely to worry about the future.  However, with the proper support, your child can live a very full and successful life, and if you learn how to give and find that support, you can put worry aside.

  • Teach your child to be independent.  Give your child the opportunity to do things for themselves whenever possible.  Be alert to your child’s unique strengths and challenges, and find ways to foster self-sufficiency in everyday tasks and situations.
  • Don’t be isolated.  Stay involved in social activities and events.  Allow your child to be a part of the wider world, where they can learn and explore.
  • Plan ahead.  Think about the skills your child will need in adulthood, especially if you expect your child to live independently eventually.  Then, begin building those skills as early as possible, and plan to develop a support system that will be there for your child in the years ahead.

Many people with Down syndrome also suffer from other brain-related issues, a condition called co-morbidity.  Here are a few of the disorders commonly associated with DS:

Mosaic Down Syndrome Brain Science

Some research has focused on the differences between mosaic Down syndrome and trisomy 21.  In particular, scientists are interested in discovering whether people with mosaic DS have milder symptoms when they have fewer cells with extra chromosomes.

The severity of symptoms varies widely between cases of mosaic DS.  Scientists believe several factors might account for the variation, including:

  • Some people have more cells with extra chromosomes.
  • Affected cells may be more numerous in different parts of the body in different people.
  • The affected cells might have subtle genetic differences from case to case.

In general, researchers have not found consistent evidence that mosaic DS (or having a lower number of trisomic cells) reliably produces milder symptoms than trisomy 21.  Some studies have suggested that people with mosaic DS are less likely to have complications such as heart problems and that people with less severe physical symptoms have milder intellectual impairment.  However, other studies have contradicted these findings.

Mosaic Down Syndrome Research

Title: Evaluating Cognitive Outcomes in Down Syndrome (ECODS-2)

Stage: Recruiting

Principal investigator: Deborah Fidler, PhD

Colorado State University

Fort Collins, CO

As basic and behavioral science identify new ways to improve cognition and behavior in individuals with Down syndrome (DS), the lack of rigorous outcome measures represents a challenging problem for interpreting findings.  Null findings in clinical trials could result from insensitive outcome measures rather than the ineffectiveness of treatment.


The long-term goal is to improve the measurement of outcomes for children and adults with DS.  Towards that goal, the investigators propose to test and refine a battery of cognitive measures that can be used in treatment studies focused on school-aged children and adults with Down syndrome.  The batteries are designed to assess key domains of the DS phenotype where gaps remain in outcome measures, including attention, executive function, learning and memory, processing speed, and social cognition.

The investigators will examine the psychometric properties of measures (test-retest, validity, sensitivity to change) and evaluate differences in the psychometric properties of measures as a function of variations in participant age, gender, degree of ID, and the participants’ physical health and medical co-morbidities.  The investigators will evaluate at least 80 children and 50 adults with Down syndrome, per site, at five time points to evaluate key domains with a diverse and novel range of methods.  This proposal aims to provide a preliminary evaluation to support the enhancement of clinical outcome measures, which ultimately will increase the accuracy in documenting improvements in the lives of children and young adults with Down syndrome.


Title: Predictors of Speech Ability in Down Syndrome

Stage: Not Yet Recruiting

Principal investigator: Stephen Camarata, PhD

Vanderbilt University Medical Center

Nashville, TN

Speech is a critical aspect of the human experience and usually develops in a “seemingly automatic process that continues from birth through adolescence and underlies many related abilities” (e.g., language and reading, see National Academy of Medicine Report on Speech and Language Disorders, 2016).  Many individuals with Down Syndrome (Trisomy 21, DS) struggle to communicate and participate more fully in human communication and educational learning experiences because their speech is difficult to understand.  The purpose of the proposed project is to measure speech-articulation accuracy and speech intelligibility and their proposed primary predictors at study entry in 20 children with DS age 6;0 to 13;11).  A validated treatment, speech recast intervention (see Yoder, Camarata & Woynaroski, 2016), will be used to drive growth in speech intelligibility as a means of evaluating changes in potential sequelae of change.  This integrated study will include measures of speech-articulation accuracy, speech-prosody, general cognitive ability, receptive vocabulary skills, and a clinical assay of oral-motor ability as potential predictors of speech intelligibility growth in DS.  The Investigators will also be measuring suprasegmental and rhythmic factors associated with growth.


Title: Developing a Down Syndrome Health Instrument

Stage: Recruiting

Principal investigator: Stephanie L. Santoro, MD, MGH

Massachusetts General Hospital

Boston, MA

Although over 200,000 individuals with DS live in the United States, studies to date have focused on outcomes apart from health.  The foundation for this proposal is based on the need to accurately measure the health of all individuals – specifically, with DS – and the absence of available tools for this population.  Creating such an instrument will provide a barometer of the current state of health for DS and hold use in future research.  This project proposes to create an instrument that directly assesses health in DS – the Down syndrome Health Instrument (DHI).  More specifically, the aims of this proposal are:

  1. To conduct focus groups among caregivers, individuals with DS, panels of experts on DS and primary care physicians, and cognitive interviews to refine a conceptual model of health for DS and create an item pool. 
  2. To administer the DHI and establish internal validity, reliability, and external validity of the DHI for use in clinical research.
  3. To test the usability of the DHI in two pilot settings: research and clinical.  

This instrument will measure patient-reported health in DS for the first time and allow measurement of health as an outcome that is not currently possible in this population.  This can identify gaps in care then direct and optimize interventions to improve care.

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