The following information may help to address your question:
What is Klinefelter syndrome?
Klinefelter syndrome (KS)
is a condition that occurs in males when they have an extra X chromosome
. Some males with KS have no obvious signs or symptoms while others may have varying degrees of cognitive, social, behavioral, and learning difficulties. Adults with Klinefelter syndrome may also experience primary hypogonadism
(decreased testosterone production), small testes
, enlarged breast tissue
), tall stature, and/or infertility. KS is not inherited
, but usually occurs as a random event during the formation of reproductive cells
(eggs and sperm). Treatment is based on the signs and symptoms present in each person.
Last updated: 10/13/2015
What causes Klinefelter syndrome?
Klinefelter syndrome usually occurs as a random event during the formation of reproductive cells
(eggs and sperm). An error in cell division called nondisjunction
results in a reproductive cell with an abnormal number of chromosomes
. For example, an egg or sperm cell may gain one or more extra copies of the X chromosome
as a result of nondisjunction. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have one or more extra X chromosomes in each of the body's cells.
Most often, Klinefelter syndrome is caused by a single extra copy of the X chromosome, resulting in a total of 47 chromosomes per cell. Males normally have one X chromosome and one Y chromosome
in each cell (46, XY), while females have two X chromosomes (46, XX). People with Klinefelter syndrome usually have two X chromosomes and one Y chromosome (47, XXY). Some people with Klinefelter syndrome have the extra X chromosome in only some of their cells; these people are said to have mosaic
It is estimated that about half of the time, the cell division error occurs during development of the sperm, while the remainder are due to errors in egg development. Women who have pregnancies after age 35 have a slightly increased chance of having offspring with this syndrome.
The features of Klinefelter syndrome are due to the extra copies of genes
on the extra X chromosome, which can alter male sexual development.
Last updated: 10/12/2015
What does it mean to have a mosaic form of Klinefelter syndrome?
Mosaicism occurs when an individual has more than one cell population with a different genetic make-up. This can include any cells within the body including blood cells, egg and sperm cells, and skin cells. Mosaicism
is present from birth.
The mosaicism present in Klinefelter syndrome is related to copies of the X and Y chromosomes (sex chromosomes
). In Klinefelter syndrome, there is an error in cell division that occurs after fertilization, usually resulting in an additional copy of the X chromosome (46, XXY). Some people with Klinefelter syndrome have an extra X chromosome only in some of their cells (46, XY/46, XXY).
Other rarer chromosomal complements resulting in mosaic Klinefelter syndrome include: 46, XX; 48, XXYY; 48, XXXY; 49, XXXYY; and 49, XXXXY.
Individuals with mosaic Klinefelter syndrome may have more mild signs and symptoms, depending on how many cells have an additional copy of the X chromosome.
Last updated: 3/28/2016
What is the incidence of mosaic Klinefelter syndrome?
Approximately 15-20% of cases of Klinefelter syndrome are mosaic.
The true prevalence is suspected to be greater as there could be cases missed given the varying levels of mosaicism
that can be found in different tissues and the potential for males with mosaic Klinefelter syndrome to have more mild symptoms and miss diagnosis.
Last updated: 3/28/2016
Where can I find more information about mosaic Klinefelter syndrome?
You can find relevant articles on mosaic Klinefelter syndrome through PubMed, a searchable database of biomedical journal articles. Although not all of the articles are available for free online, most articles listed in PubMed have a summary available. To obtain the full article, contact a medical/university library or your local library for interlibrary loan. You can also order articles online through the publisher’s Web site. Using "mosaic+Klinefelter syndrome" as your search term should help you locate articles. Use the advanced search feature to narrow your search results. Click here to view a search.
The National Library of Medicine (NLM) can help you find libraries in your area where you can get the full text of medical articles. The webpage also describes how you can get these articles through interlibrary loan and Loansome Doc (an NLM document-ordering service). You can search for libraries at the following link http://nnlm.gov/members/
. You can also contact the NLM toll-free at 1-888-346-3656.
Last updated: 3/28/2016
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