A tetrasomy is a form of aneuploidy with the presence of four copies, instead of the normal two, of a particular chromosome.

Full tetrasomy of an individual occurs due to non-disjunction when the cells are dividing (meiosis I or II) to form egg and sperm cells (gametogenesis). This can result in extra chromosomes in a sperm or egg cell. After fertilization, the resulting fetus has 48 chromosomes instead of the typical 46.

• Cat eye syndrome where partial tetrasomy of chromosome 22 is present
• Pallister-Killian syndrome (tetrasomy 12p)
• Tetrasomy 9p
• Tetrasomy 18p
• Tetrasomy 21, a rare form of Down syndrome

• Tetrasomy X
• XXYY syndrome