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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]
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]
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.).
Robertsonian translocations can only occur between chromosomes which have the centromere very close to one end. This means these chromosomes have a long arm which is particularly long, and a short arm which is particularly short. These are known as acrocentric chromosomes. Humans have five of these acrocentric chromosomes: 13, 14, 15, 21 and 22.
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Many trisomic conditions result in stillborn infants (major exceptions include trisomies 13, 18, 21). Trisomic rescue may be a natural means to keep a fetus as viable as possible (though uniparental disomy may occur and result in syndromes such as Prader-Willi and Angelman syndromes due to genetic imprinting ).