The exchange of corresponding segments between chromatids of homologous chromosomes. The process is inferred genetically from the recombination of linked genes and cytologically from the formation of * chiasmata. Normally crossing over is reciprocal (equal) but unusual segregation products in ordered tetrads indicate that it can very occasionally be nonreciprocal (unequal). Work with imperfect (asexual) fungi indicates that crossing over can also occur during mitosis. Chromosome reassortment and crossing over together constitute the main source of genetic variation in sexually reproducing organisms. Crossing over is suppressed by chromosome inversions. See also independent assortment, parasexual recombination, tetrad analysis. |
