The incidence of paracentric inversions in the general population has not been clearly established, it ranges from 0.09 to 0.49/1,000. Homologue pairing during meiosis in a paracentric-inversion heterozygote is maximized by the formation of an inversion loop. If a crossing-over occurs within this loop, dicentric and acentric chromosomes are formed. Resulting gametes can have variety of duplications and deficiencies and give a non-viable progeny. One of the exceptions to the rule is a mutation event „U loop recombination“. FromU-loop event a monocentric recombinant chromosome can arise by an abnormal process, which involves chromatid breakage and reunion. Most of the paracentric inversions inman are harmless and the risk of heterozygotes having a child with an unbalanced karyotype is relatively low. In carriers of an accidentally discovered paracentric inversion, amniocentesis is optional. However, in some cases, it is difficult to distinguish between a paracentric inversion and paracentric insertion - the risk of the insertion is about 15 %. When a de novo inversion is detected in amniotic fluid, the overall risk for two-break rearrangements is 6.7 %.

        Key words: chromosomes, paracentric inversion, recombination, prenatal diagnosis.