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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.
Females with one affected X chromosome and one normal X chromosome tend to have milder symptoms. Unlike many other types of intellectual disability, the genetics of these conditions are relatively well understood. [2] [3] It has been estimated there are ~200 genes involved in this syndrome; of these ~100 have been identified. [4]
The cause of the condition lies in conception or utero with the aggregation of two distinct zygotes or blastocysts (one of which expresses 46,XX and the other of which expresses 46,XY) into a single embryo, [4] which subsequently leads to the development of a single individual with two distinct cell lines, instead of a pair of fraternal twins.
A disease or trait determined by a gene on the X chromosome demonstrates X-linked inheritance, which can be divided into dominant and recessive patterns. The first X-linked genetic disorder described on paper was by John Dalton in 1794, then later in 1910, following Thomas Hunt Morgan's experiment, more about the sex-linked inheritance was ...
X-linked recessive inheritance. X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation, see zygosity.
X-linked severe combined immunodeficiency (X-SCID) is an immunodeficiency disorder in which the body produces very few T cells and NK cells. In the absence of T cell help, B cells become defective. [1] It is an X-linked recessive inheritance trait, stemming from a mutated (abnormal) version of the IL2RG gene located on the X-chromosome.
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. Cri du chat syndrome – (French for "cry of the cat" after the persons' malformed larynx ) a partial monosomy caused by a deletion of the end of the short arm of chromosome 5
Trisomy X, also known as triple X syndrome and characterized by the karyotype [note 1] 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 females, but is rarely diagnosed; fewer than 10% of those with the condition know they have it.