A consequence of heterokaryosis is the parasexual cycle, discovered in Aspergillus nidu-lans by G. Pontecorvo (see Davis, 2003). The parasexual cycle involves the fusion of two genetically unlike nuclei to form a transient diploid nucleus. During subsequent multiplication of the diploid nucleus, a rare event of mitotic crossing-over occurs and members of each homologous pair of chromosomes assort independently of other pairs. The conidia produced during the cycle are genetically different from the original mycelium as a result of the crossover. The parasexual cycle is not a common phenomenon; however, the general approach of parasexual recombination worked out in A. nidulans provided the model for assigning human genes to chromosomes using somatic cell hybrids.
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