title: Tetrasomy
text: # Tetrasomy
  
  A tetrasomy is a form of aneuploidy with the presence of four copies, instead of the normal two, of a particular chromosome.
  
  ## Causes
  
  ### Full
  
  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.
  
  ### Autosomal tetrasomies
  
  * 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
  
  ### Sex-chromosome tetrasomies
  
  * Tetrasomy X
  * XXYY syndrome
  