Humans typically have 22 pairs of autosomal chromosomes in our cells, and a pair of sex chromosomes. About 2.7 million individuals have an extra, 47th autosomal chromosome called a small supernumerary marker chromosome (sSMC). These small supernumerary marker chromosomes can originate from any of the 24 different human chromosomes. About 70% of the cases with sSMC are de novo (new mutations), 30% are inherited within a family.
About 30% of the carriers of a sSMC are clinically abnormal. The main problem in connection with a sSMC appears when the diagnosis of the presence of a sSMC is made prenatally. Until recently there was no possibility to make clear predictions about the outcome of the pregnancy. However, research is being carried out into the link between the presence of a sSMC in individuals and any consequent symptoms.
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