Most instances of aneuploidy result from accidental chromosome mis-segregation at meiosis (nondisjunction). The error is spontaneous and can occur in any particular meiocyte at the Frst or second division. In humans there is a maternal-age effect associated with nondisjunction of chromosome 21, resulting in a higher incidence of Down syndrome in the children of older mothers. The other general category of chromosome muta-tions is structural rearrangements, which include dele-tions, duplications, inversions, and translocations. Chro-mosomal rearrangements are an important cause of ill health in human populations and are useful in engi-neering special strains of organisms for experimental and applied genetics. In organisms with one normal chromosome set plus a rearranged set (heterozygous rearrangements), there are unusual pairing structures at meiosis resulting from the strong pairing affinity of ho-mologous chromosome regions. ±or example, heterozy-gous inversions show loops, and reciprocal transloca-tions show cross-shaped structures. Segregation of these structures results in abnormal meiotic products
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