Chromosome 20 trisomy

Alternate names

Trisomy chromosome 20; Trisomy 20; Trisomy 20 mosaicism; Mosaic trisomy 20

Definition

Chromosome 20 trisomy, (also called trisomy 20) is a condition in which a fetus or individual has an extra full or partial copy of chromosome 20 in some or all of of his/her cells.

An example of Trisomy

Summary

• An extra full copy of chromosome 20 in all of a person's cells is rare, and almost all fetuses with this do not survive past the first trimester of pregnancy.
• The presence of an extra copy of only part of chromosome 20 is called partial trisomy 20; and an extra copy of chromosome 20 in only some of a person's cells is called mosaic trisomy 20.
• Mosaic trisomy 20 is the most common type of chromosome 20 trisomy and is one of the more common chromosomal abnormalities found during prenatal diagnostic testing.

Cause

• Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops.
• It is not always known why these errors occur, but it is thought that nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in a fetus or child.
• Egg and sperm cells should each contain 23 chromosomes.
• When they join together, they form a fertilized egg with 46 chromosomes.
• Sometimes, something goes wrong before fertilization.
• An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes.
• When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality.
• If an egg or sperm cell has 24 chromosomes because either has an extra copy of chromosome 20, this can cause trisomy 20 (the resulting embryo will have 3 copies of chromsome 20).
• In most cases, an embryo with 3 full copies of chromosome 20 in all cells does not survive and the pregnant woman has a miscarriage, often very early in pregnancy.
• Other types of errors can alter the structure of one or more chromosomes.
• Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes.
• However, small pieces of a chromosome (or chromosomes) may be duplicated, and in the case of part of chromosome 20 being duplicated, it would be called partial trisomy 20 (because there are 3 copies of only part of chromosome 20 in each cell).
• Errors in cell division involving chromosome 20 can also occur soon after fertilization, which can cause mosaicism, a condition in which an individual has cells with different genetic makeups.
• Individuals with the mosaic form of trisomy 20 have an extra chromosome 20 in some, but not all, of their cells.
• Some individuals with chromosomal mosaicism may be mildly affected or apparently not affected at all, but the severity of the condition may depend on the number of abnormal cells that are present.
• Other individuals may be severely affected.

Inheritance

• Most cases of chromosome 20 trisomy are not inherited and result from random events during the formation of eggs and sperm in healthy parents.
• An error in cell division, called nondisjunction, results in an egg or sperm cell with an abnormal number of chromosomes.
• If an egg or sperm cell gains an extra copy of chromosome 20, the embryo will have an extra chromosome 20 in each cell of the body.
• For more information about the causes of different types of chromosome 20 trisomy (full, partial or mosaic trisomy 20)
• Translocation trisomy 20 can be inherited.
• A healthy person (that does not have trisomy 20) can carry a rearrangement of genetic material between chromosome 20 and another chromosome (called a translocation).
• These rearrangements are called balanced translocations because there is no extra material from chromosome 20.
• A person with a balanced translocation involving chromosome 20 has an increased chance of passing extra material from chromosome 20 to his/her child, which would result in a form of trisomy 20 in the child.

Signs and symptoms

• Mosaic trisomy 20 is the most common type of chromosome 20 trisomy and is one of the more common chromosomal abnormalities found during prenatal diagnostic testing.
• Studies have shown that the child is normal in the vast majority of prenatally diagnosed individuals.
• However, features that have been reported include spinal abnormalities (including spinal stenosis, vertebral fusion, and kyphosis), hypotonia (decreased muscle tone), lifelong constipation, sloped shoulders, and significant learning disabilities despite normal intelligence.

Diagnosis

Chromosome disorders may be suspected in people who have developmental delays, intellectual disabilities and/or physical abnormalities.

Several types of genetic tests can identify chromosome disorders:

• Karyotype - a karyotype is a laboratory test that produces an image of a person's chromosomes. This test can be used to diagnose large deletions.
• FISH - a laboratory technique that is used to detect and locate a specific DNA sequence on a chromosome. During FISH, a chromosome is exposed to a small DNA sequence called a probe that has a fluorescent molecule attached to it. The probe sequence binds to its corresponding sequence on the chromosome. This test can be used in combination with karyotyping for deletions that are too small to be seen on karyotype, alone. However, FISH is only useful if the person ordering the test suspects there is a deletion of a specific region of 4q.
• Array CGH - a technology that detects deletions that are too small to be seen on karyotype.

Treatment

Treatment is based on the signs and symptoms present in each person.

NIH genetic and rare disease info

• NIH info on Chromosome 20 trisomy

Chromosome 20 trisomy is a rare disease.

Chromosome 20 trisomy Resources
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