Approximately 80 percent of those with 47,XXY have the 47,XXY genetic signature. In the other 20 percent, there may be more X and Y chromosomes, with genetic signatures of 48,XXXY, 48,XXYY, and 49,XXXXY. Or, individuals may have a mix of cells within the body, some XY and others XXY, which is called mosaicism. Some individuals have a genetic signature of 46,XX, but have a tiny piece of the Y chromosome appended to one of the X chromosomes, giving them male genitalia and secondary sex characteristics, such as facial hair and male muscle development.47,XYY is incorrectly associated as a variant of 47,XXY, but it is actually a distinctly different sex chromosome aneuploidy. The most obvious difference is that 47, XYY males have none of the medical and infertility issues that individuals with 47,XXY may have, although there can be behavioral difficulties and learning disabilities associated with XYY.