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Turner syndrome (TS), commonly known as 45,X, or 45,X0, [note 1] is a chromosomal disorder in which female cells have only one X chromosome instead of two, 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.
The embryo, then the fetus, and then eventually the baby will have what is known as a 45,X/46,XY constitution. There are many chromosomal variations that cause the 45,X/46,XY karyotype, including malformation (isodicentricism) of the Y chromosomes, deletions of Y chromosome or translocations of Y chromosome segments. [ 2 ]
Most individuals with Turner syndrome have one X and no Y chromosome. However, about 5.5% of Turner syndrome individuals have an sSMC containing part of a Y chromosome. This partial Y chromosome-bearing sSMC may include the SRY gene located on the p arm of the Y chromosome at band 11.2 (notated as Yp11.2).
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.
Turner syndrome (Ullrich-Turner syndrome and gonadal dysgenesis) – a condition that describes a female born with only one X chromosome or with an abnormal X chromosome, making her karotype 45,X0. It occurs in 1 in 2,000 to 5,000 females. [ 61 ]
XY complete gonadal dysgenesis, also known as Swyer syndrome, is a type of defect hypogonadism in a person whose karyotype is 46,XY. Though they typically have normal vulvas, [1] the person has underdeveloped gonads, fibrous tissue termed "streak gonads", and if left untreated, will not experience puberty.
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 .
Rather than having monosomy, or only one copy, the majority of aneuploid people have trisomy, or three copies of one chromosome. [citation needed] An example of trisomy in humans is Down syndrome, which is a developmental disorder caused by an extra copy of chromosome 21; the disorder is therefore also called "trisomy 21". [7]