A tetrasomy is a form of aneuploidy

Aneuploidy

Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...

 with the presence of four copies, instead of the normal two, of a particular chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

.

Full

Full tetrasomy of an individual occurs due to non-disjunction when the cells are dividing

Cell division

Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

 (meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 I or II) to form egg and sperm cells (gametogenesis

Gametogenesis

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes,...

). This can result in extra chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s in a sperm or egg cell. After fertilization, the resulting fetus has 48 chromosomes instead of the typical 46.

Autosomal tetrasomies

• Cat eye syndrome
  Cat eye syndrome
  Cat Eye Syndrome is a rare condition caused by the short arm and a small section of the long arm of human Chromosome 22 being present three or four times instead of the usual two times. The term "Cat Eye" syndrome was coined because of the particular appearance of the vertical colobomas in the...
  
   where tetrasomy of chromosome 22 is present
• Pallister-Killian syndrome
  Pallister-Killian syndrome
  Pallister–Killian syndrome is an extremely rare genetic disorder occurring in humans. Pallister-Killian occurs due to the presence of the anomalous extra isochromosome 12p, the short arm of the twelfth chromosome. This leads to the development of tetrasomy 12p...
  
   (tetrasomy 12p)
• Tetrasomy 9p
  Tetrasomy 9p
  Tetrasomy 9p is a rare genetic disease due to the inclusion of an isochromosome of the short arm of chromosome 9 . It is generally a mosaic condition.-External links:*...
  
  
• Tetrasomy 18p
  Tetrasomy 18p
  Tetrasomy 18p is a genetic condition that is caused by the presence of an isochromosome, composed of two copies of the short arm of chromosome 18. It is characterized by multiple medical and developmental concerns.- Genetic Basis :...
  
  

Sex-chromosome tetrasomies

• 48, XXXX syndrome
• 48, XXYY syndrome
• Klinefelter's syndrome
  Klinefelter's syndrome
  Klinefelter syndrome, 46/47, XXY, or XXY syndrome is a condition in which human males have an extra X chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome...
  
  , where XXXY tetrasomy is present