<unk>  is caused by two extra copies of the Y chromosome, the chromosome that determines male sexual development. Sex chromosome aneuploidies are the most frequent form of aneuploidy in humans. Though a 48-chromosome complement involving the autosomes would be unsurvivable, 48,XYYY and other high-level sex chromosome aneuploidies such as XXXY syndrome and tetrasomy X—or indeed 49-chromsome disorders such as pentasomy X—are survivable with relatively mild phenotypes due to the paucity of genes vital to basic development on the sex chromosomes. Sex chromosome aneuploidies occur via a process known as non-disjunction, where chromosomes fail to divide properly during cell division and produce gametes, in this case sperm, with an abnormal number of chromosomes. In the case of<unk> , the karyotype may be a result of non-disjunction with an XYY father, or of double non-disjunction resulting in a YYY sperm with a chromosomally normal father. Non-disjunction can also arise during embryo development shortly after conception, which often gives rise to mosaicism.
XYYY syndrome
XYYYY syndrome
XYY syndrome
XXYY syndrome
XX male syndrome
XXXY syndrome
Turner syndrome
Klinefelter syndrome