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Mosaic Down syndrome

What is Mosaic down syndrome

It is a mixture of two types of cells. Many people have 46 chromosomes while others have 47, which means they have extra chromosomes 21.

Mosaic down syndrome is also known as mosaicism Down syndrome. In a chromosome study, a total of 20 different cells are analyzed. A diagnosis of mosaic Down syndrome is confirmed if the baby has the following characteristics:

  • At least five of the 20 cells have 46 chromosomes while the rest have 47 chromosomes secondary to excess chromosome 21.
  • The level of mosaicism is 75%, but the percentage varies in different parts of the body.
  • Babies with mosaic Down syndrome have the same clinical manifestations as babies with trisomy 21. However, the former has fever characteristics of Down syndrome than the latter. (1, 2, 3)

A photo of a child with mild mosaic down syndrome.image

Photo 1: A photo of a child with mild mosaic down syndrome.
Picture Source: 2.bp.blogspot.com

A female child with mosaic down syndrome.photo

Image 2: A female child with mosaic down syndrome.
Photo Source: syndromespedia.com

A boy with trisomy 21 chromosomal defect.image

Photo 3: A boy with trisomy 21 chromosomal defect.
Picture Source: images.wisegeek.com

How mosaic Down syndrome happens?

Ideally, the union of egg and sperm cells leads to a single cell with a total of 46 chromosomes. They are duplicate chromosomes, which will undergo division (two identical daughter cells). The daughter cells will divide forming four cells.

Cell division process will then again take place. Normal cell division and replication process leads to the same number of chromosomes in a fertilized egg. However, not all replication process is normal. An abnormality in the process can lead to chromosomal defects.

It could be that at the time of conception, there is an excess copy of chromosome #21. During the replication and division process, an error occurred. A copy of chromosome #21 was not successfully passes onto both cells. As a result, a second cell group was formed with only 46 chromosomes.

What is a non-mosaic down syndrome? It is a condition wherein all the cells in a patient’s body have extra chromosomes. With mosaic Down syndrome, only a portion of the cell have extra chromosomes. (4, 6, 7)

Difference between Mosaic Down syndrome and Down syndrome

The body contains numerous cell types and these cells originate from a single fertilized egg (zygote). In a down syndrome (full trisomy 21), the error happens on either the egg or sperm cell as they unite to form a zygote. The error occurs at the start of the development of the zygote.

As a result, all cells that come from the zygote will have extra chromosome #21. In a mosaic down syndrome, the error takes place after the egg is fertilized. Mosaic Down syndrome characteristics are somewhat the same as the trisomy 21, but trisomy 21 has more pronounced clinical features.

Mosaic Down syndrome symptoms are mild because patients still have cells with normal number of chromosomes. (2, 3, 5)

Mosaic Down syndrome Symptoms

  • The back of the head is flat and aligned with the neck.
  • There is an excess skin around the neck.
  • The hair is fine and straight.
  • The ears are small and low set.
  • The nasal bridge is flat.
  • You will notice a skin fold in the inside corner of the eyes (epicanthal folds).
  • There is a white fleck on the colored portion of the eyes (brush field spot). (8)
  • The eruption of baby and adult teeth is delayed.
  • The tongue is protruded.
  • The fingers and toes are short and there is a single crease across one of both palms.
  • There is an abnormally large space between the first and second toes.
  • The skin is dry and the muscle tone at birth is low.
  • Short stature. (6)
  • The eyes slant upward.
  • The body is extremely flexible.
  • Almond shaped eyes. (9)

The intellectual development of people with mosaic Down syndrome

The average IQ score of people with mosaic Down syndrome is between 10 and 30, which is higher than people with Down syndrome (trisomy 21).

How common is mosaic Down syndrome?

Mosaic Down syndrome is a rare condition. It usually affects two to three patients in every 100 children diagnosed with Down syndrome (trisomy 21). Chromosomal abnormalities are most likely to happen in women who have late pregnancy.

Is Mosaic Down syndrome hereditary? It is not inherited. It occurs during cell division early in fetal development. (10, 11)

Mosaic Down Diagnosis

A genetic testing using the baby’s blood helps confirm the diagnosis of mosaic Down syndrome. About 20 to 25 cells are examined. A diagnosis of mosaic Down syndrome is confirmed if some of the patient’s cells have trisomy 21 and others don’t.

Mosaic Down syndrome late diagnosis can be quite difficult to confirm because mosaicism can also occur across cell lines. If you are suspecting mosaic Down syndrome but couldn’t accurately confirm it through blood test, then additional tests will be performed such as the skin and bone marrow testing.

Medical experts strongly believed that the skin cell test represents the chromosomal composition of the brain cells. An amniocentesis or chorionic villus sampling during pregnancy can help detect mosaic Down syndrome. (1, 4, 8, 11)

Mosaic Down Syndrome Treatment

Mosaic Down syndrome is a genetic disorder. It does not have cure or treatment. Once you are diagnosed with mosaic Down syndrome, you will have it throughout your lifetime. What can be treated though are the complications brought by the disorder. The goal is to improve the quality of life of patients with mosaic Down syndrome.

Patients with MDS usually complain of joint pain. To alleviate the pain and discomfort, the patient is put on a physiotherapy. The patient should have a periodic medical check up to detect any abnormalities and treat them at an early stage.

Regular dental check-up is strongly recommended to monitor the growth of teeth. Patients with mosaic Down syndrome are prone to obesity. Hence, a healthy diet along with regular exercise are strongly recommended. (4, 6, 12)

Life expectancy

People with mosaic Down syndrome can live up to 66 year old. The condition may sound alarming but in reality, it is not a fatal condition.

To improve the quality of life and lengthen the life expectancy of the patient, any medical complications should be treated right away. If they are left untreated, they can be potentially fatal. (5, 6)


  1. www.stanfordchildrens.org
  2. www.mosaicdownsyndrome.com
  3. www.imdsa.org
  4. www.ds-health.com
  5. www.down-syndrome.org
  6. www.verywell.com
  7. www.urmc.rochester.edu
  8. https://en.wikipedia.org
  9. https://patient.info
  10. https://syndromespedia.com
  11. www.disabilitybenefitscenter.org
  12. www.mosaicdownsyndrome.org

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