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Turner syndrome (TS), commonly known as 45,X, or 45,XO, [note 1] is a chromosomal disorder in which cells have only one X chromosome or are partially missing an X chromosome (sex chromosome monosomy) leading to the complete or partial deletion of the pseudoautosomal regions (PAR1, PAR2) in the affected X chromosome.
Human conditions due to monosomy: Turner syndrome – Females with Turner syndrome typically have one X chromosome instead of the usual two X chromosomes. Turner syndrome is the only full monosomy that is seen in humans — all other cases of full monosomy are lethal and the individual will not survive development.
About half of individuals with Turner syndrome have monosomy X (45,X), which means each cell in a woman's body has only one copy of the X chromosome instead of the usual two copies. Turner syndrome can also occur if one of the sex chromosomes is partially missing or rearranged rather than completely missing.
Monosomy: Monosomy refers to lack of one chromosome of the normal complement. Partial monosomy can occur in unbalanced translocations or deletions, in which only a portion of the chromosome is present in a single copy (see deletion (genetics)). Monosomy of the sex chromosomes (45,X) causes Turner syndrome. 2: Disomy
The only non-lethal full monosomy occurring in humans is the one causing Turner's syndrome. Around 30% of Turner's syndrome cases demonstrate mosaicism, while complete monosomy (45, X) occurs in about 50–60% of cases. Mosaicism isn't necessarily deleterious, though.
45,X/46,XY mosaicism, also known as X0/XY mosaicism and mixed gonadal dysgenesis, [1] is a mutation of sex development in humans associated with sex chromosome aneuploidy and mosaicism of the Y chromosome.
Monosomy, with the presence of only one chromosome (instead of the typical two in humans) from a pair, which affects chromosome 14. Fetuses with monosomy 14 are not viable. [ 1 ] Only mosaic cases exist and these usually present with severe symptoms such as intellectual disability , ocular colobomata , microcephaly , and seizures .
Polysomy X may cause mental and developmental retardation and physical malformation. Klinefelter syndrome is an example of human polysomy X with the karyotype 47, XXY. X chromosome polysomies can be inherited from either a single maternal (49, X polysomies) or paternal (48, X polysomies) X chromosome. [18]