mosaicism involving sex chromosomes

First, some background: A karyotype is a picture of the chromosomes in a cell. A karyotype is used to see what kinds of chromosomes a person has. It is created by taking a blood or tissue sample from a person, and then staining the chromosomes with dye and photographing them through a microscope. The photograph is then cut up and rearranged so that the chromosomes are lined up into corresponding pairs. The result is usually reported as the number and type of a person’s chromosomes, such as 45,X (the individual has 22 pairs of matched chromosomes and one X chromosome, also known as Turner Syndrome); 46,XX (the individual has 22 pairs of matched chromosomes and two X chromosomes); 46,XY (the individual has 22 pairs of matched chromosomes, one X chromosome and one Y chromosome); 47, XXY.

A person is said to have a “mosaic karyotype” when he or she has one kind of karyotype in some of his or her cells, and a different karyotype in other cells. An example is when a person is said to have a 45,X/46,XX karyotype; that means he or she has 46,X in some cells, and 46,XX in other cells. Mosaicism happens because sometimes cells divide incorrectly early in the life of an embryo.

For instance a woman with Mosaic Turner Syndrome may have some cells that are XO (typical Turner Syndrome karyotype) and some cells that are XX (typical female karyotype). Mosaicism also occurs in milder forms of Klinefelter Syndrome called 46/47 XY/XXY mosaic. In this case, the XY cells would have 46 chromosomes (a typical number of chromosomes) and the XXY cells would have 47 chromosomes.

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