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Karyotype of a human with Trisomy 21 (Down syndrome). Trisomies can occur with any chromosome, but often result in miscarriage rather than live birth.For example, Trisomy 16 is most common in human pregnancies, occurring in more than 1%, but the only surviving embryos are those having some normal cells in addition to the trisomic cells (mosaic trisomy 16). [3]
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.
A karyotype of an individual with trisomy 21, showing three copies of chromosome 21.. An abnormal number of chromosomes is known as aneuploidy, and occurs when an individual is either missing a chromosome from a pair (resulting in monosomy) or has more than two chromosomes of a pair (trisomy, tetrasomy, etc.).
Also monosomies can present with some form of mosaicism. 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.
Down syndrome or Down's syndrome, [12] also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3] It is usually associated with developmental delays, mild to moderate intellectual disability, and characteristic physical features.
Chromosome 16. Trisomy 16 is a chromosomal abnormality in which there are 3 copies of chromosome 16 rather than two. [1] It is the most common autosomal trisomy leading to miscarriage, and the second most common chromosomal cause (closely following X-chromosome monosomy). [2]
After having one child with the condition, the risk of having a second is typically around one percent. [2] It is the second-most common condition due to a third chromosome at birth, after Down syndrome for a third chromosome 21. [4] Trisomy 18 occurs in around 1 in 5,000 live births. [3] Many of those affected die before birth. [3]
Complete trisomy 8 causes severe abnormalities on the developing fetus and can be a cause of miscarriage. [2] [3] Complete trisomy 8 is usually a gestational lethal condition, whereas trisomy 8 mosaicism is less severe and individuals with a low proportion of affected cells may exhibit a comparatively mild range of physical abnormalities and developmental delay. [4]