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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.
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]
Only mosaic cases exist and these usually present with severe symptoms such as intellectual disability, ocular colobomata, microcephaly, and seizures. Organ malformations are also associated due to the chromosome 14 having several genes for organ development, including heart defects, and brain problems.
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.
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.).
The International-Differences of Sex Development (I-DSD) is a research organization in Europe. This organization connects medical and research centers internationally in an effort to improve clinical practice, research, and general understanding of differences of sex development. [68]
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.
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]