iGen3 16-08

Paracentric Chromosome Inversion: effect on crossover products

A paracentric ("away from the center") inversion does not include the centromere (o). In the example, the BCD region of the standard chromosome oABCDEFGH has been inverted, to give the locus order oA(DCB)EFGH. After chromatid duplication in meiosis, the paired inverted chromosome forms a loop so that the loci pair with their homologous on the uninverted chromosome.

Occurence of a crossover event inside the loop produces four types of products: a dicentric brdige, an acentric fragment, and two chromosomes with the standard and inverted gene orders. Because it lacks a centromere and cannot be drawn to either cell pole, the acentric fragment will be lost during meiosis. Because the two centromeres of the dicentric bridge are drawn to opposite poles, the bridge will be broken mechanically at a random position, with resultant production of two duplication / deletion products in which one or more loci have been duplicated or deleted, according to where the breakage of the bridge occured with respect to the inverted region. Inheritance of one of these products will result in segmental aneuploidy.

Note that, unlike a pericentric inversion, not all gene loci are represented in the final products, because those on the acentric fragment are lost. In the final set of fragments, count the number of A versus B C D  versus E F G H loci.

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2014 by Steven M. Carr