What is Mosaicism?

J.M. Willhite

Mosaicism is a condition where an individual’s cells are genetically variegated. Also known as Gonadal or chromosomal mosaicism, this condition is congenital and can affect any type of cell. Resulting from erroneous cell division while in utero, treatment is dependent upon the type of genetic condition. Prognosis for an individual with mosaicism is dependent on how his or her organs and tissues are affected.

Those with mosaic Down syndrome have some cells with three copies of chromosome 21 and some cells that have two copies of chromosome 21.
Those with mosaic Down syndrome have some cells with three copies of chromosome 21 and some cells that have two copies of chromosome 21.

Cells that are generally affected with this condition include blood, skin, and reproductive cells. Mosaicism can manifest as a variety of syndromes such as Turner, Klinefelter, and Down. Symptoms associated with these conditions are difficult to anticipate and vary in severity.

Heart defects can be a secondary condition of mosaicism.
Heart defects can be a secondary condition of mosaicism.

Mosaicism can be diagnosed by the administration of karyotyping, or chromosome analysis. Typically each sex has 46 chromosomes, with males having an XY pairing and females an XX pairing. When an individual has a form of mosaicism, the pairing or number of chromosomes may be abnormal.

A pelvic exam may be used to diagnose mosaicism.
A pelvic exam may be used to diagnose mosaicism.

Turner syndrome is a condition that affects females and results from the absence of the usual chromosome pair of XX. Cells within the individual are missing either a portion or all of a single X chromosome. Symptoms of Turner syndrome include short height, an absence of menstruation, and a broad, flat chest. Infants with Turner syndrome may have a broad, webbed neck or distended hands and feet.

Tests conducted to confirm a diagnosis of this condition may include an echocardiogram, a pelvic exam, and an ultrasound of the kidneys and reproductive organs. Treatment for Turner syndrome may include the administration of estrogen replacement therapy and growth hormone. Complications associated with Turner syndrome include obesity, hypertension, and kidney problems.

Klinefelter syndrome is characterized by the presence of an additional X chromosome in a male with the normal XY chromosomal structure. Males with this condition may experience symptoms including small genitals, abnormally proportioned extremities, and a lack of body hair. In some cases, Klinefelter may go undiagnosed until adulthood when issues of infertility can arise.

Tests used to confirm a diagnosis of Klinefelter syndrome include a semen count and blood tests to measure male hormone levels. Treatment for this condition may include testosterone therapy. Males with Klinefelter syndrome may experience complications including depression, learning disabilities, or the development of an autoimmune disorder, such as rheumatoid arthritis or lupus.

Individuals with Down syndrome have an additional chromosome, which, in most cases, is chromosome 21. Also called Trisomy 21, this form of mosaicism is the most prevalent cause of congenital defects. Those with this condition may exhibit a range of symptoms varying in severity. Common physical indicators of this condition include a flattened nose, small features, wide hands, and an abnormally shaped head. Features of this condition extend beyond the physical to impact emotional, social, and, sometimes, psychological development.

The delayed development that children with Down syndrome experience can contribute to issues such as learning disabilities, impulsive behavior, and an impaired attention span. Frustration and anger over their delayed development can manifest in behavioral problems, including aggressiveness. Secondary conditions including hypothyroidism, heart defects, and joint problems have been known to occur.

An initial diagnosis of Down syndrome is generally made at birth, but additional tests may be administered to confirm. Tests including an echocardiogram, blood tests for chromosomal analysis, and x-rays of the chest are usually employed. Individuals with this form of mosaicism must be monitored as a precautionary measure to screen for the development of secondary conditions. Though there is no treatment for Down syndrome, individuals with congenital defects may require corrective surgery to restore and promote functionality. Complications associated with this condition include hearing and sight issues, heart and gastrointestinal problems, and sleep apnea.

Down syndrome is considered a type of mosaicism.
Down syndrome is considered a type of mosaicism.

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