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A genetic or chromosomal variation that does not occur in all body cells. The proportion of normal to anomalous cells will determine the severity of the variation. A mosaic XXY would have some cell lines which are XY and some that are XXY or variant.
"Different degree of mosaicism: That is the percentage of cells in your body that have the extra chromosome. Even when they do a blood test and tell you that "100% of the cells are XXY", what they are saying is "1005 of the cells from your blood that we tested have XXY". They may have missed some cells without an X chromosome, and they do not (know) if the cells in your brain, in your skin, in your gut etc. etc. have all XXY. The concept is that there are men with 100% XXY in all their cells, and some others that have a variable percentage."
- by Dr. Arturo Rolla, from the KS&A XXY+ADULTS email list, Feb. 28, 1999. The full text of Dr. Rolla's letter can be found at Documents
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