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