Mosaicism flipbook

WEB’S MOST

FREQUENTLY ASKED QUESTIONS ABOUT MOSAICISM

What is mosaicism?

Mosaicism is the presence of two or more genetically different sets of cells within the same person.

In this example, some cells contain a total of 46 chromosomes, while others have a total of 47 chromosomes.

How does mosaicism occur?

Mosaicism can take place during embryonic development or later in life, as a result of:

Cell division errors X-inactivation DNA changes

Chromosomal aneuploidy (change in number of chromosomes) Epigenetic changes

Mosaicism and chimerism

Like mosaicism, chimerism is also the presence of two or more genetically different sets of cells within an individual.

Mosaicism

Zygote

However, chimerism occurs when two or more zygotes are fused together.

Chimerism

Zygote 1

Zygote 2

What is germline mosaicism?

MOSAICISM

OCCURS INDIVIDUAL HAS SETS OF CELLS

“

…the gonads (ovaries or testes). This means that certain sperm or oocytes have different number of cells than others. While the individual with the germline mosaicism is phenotypically normal, they can have children who are affected if they inherit the mosaic sperm or oocyte.

Some disorders, like Duchenne muscular dystrophy and Osteogenesis imperfecta are particularly prone to germline mosaicism. Additionally, approximately 5% of Down syndrome cases are caused by germline mosaicism.

What is somat ic mosaicism?

“ MOSAICISM THAT WHEN AN HAS DIFFERENT CELLS IN…

…populations other than the gonads, also known as in the ‘soma’.

An individual with somatic mosaicism may or may not exhibit symptoms. A lot of different factors determine whether and in what severity mosaicism will affect an individual. Somatic mosaicism cannot be transmitted to offspring. Somatic mosaicism is much more common than germline mosaicism.

Mosaicism symptoms

Mosaicism may have no effect, or affect a person only mildly. It is also associated with genetic disorders, aging, and miscarriages. Symptoms vary and are difficult to predict as they depend on a lot of factors, such as:

Timing of mosaicism and number of cells affected

Mocaicism originating in the early post-zygotic stage will affect more populations of cells and is likely to have more severe consequences than mosaicism originating later in the embryonic, fetal, or postnatal stage.

Affected chromosome and genes

The symptoms and severity of mosaicism depend on the affected chromosome, the relevant genes and their associated roles.

Population of affected cells

Mosaicism can affect the entire organism, or be specific to a single organ or tissue. For example, placental mosaicism is a phenomenon where the placenta is mosaic while the fetus is not.

What is an example of mosaicism?

Mosaic Down syndrome

Down syndrome is a condition where all the cells in the body have 3 copies of chromosome 21, instead of 2.

Mosaic Down syndrome means that the individual has 3 copies of chromosome 21 in some of their cells, and 2 copies of chromosome 21 in other cells.

About 1 or 2% of people with Down syndrome have mosaic Down syndrome.

Mosaic Turner syndrome

Turner syndrome is a condition where all the cells in the body are either partially, or completely missing an X chromosome. About 1 in 2,000 babies have Turner syndrome.

Mosaic Turner syndrome means that the individual has two X chromosomes in some of their cells, and a part or the entire 2nd X chromosome is missing in other cells.

About 30% of people with Turner syndrome have mosaic Turner syndrome.