Tetrasomy

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

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 tetrasomy of chromosome 22 is present
 * Pallister-Killian syndrome (tetrasomy 12p)
 * Tetrasomy 9p
 * Tetrasomy 18p

Sex-chromosome tetrasomies

 * 48, XXXX syndrome
 * 48, XXYY syndrome
 * Klinefelter's syndrome, where XXXY tetrasomy is present