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Modern Genetic Analysis Chapter 5. Recombination of Genes
Mitotic Crossing-Over

It was a surprise for geneticists to discover that crossing-over can also occur at mitosis. Presumably it must take place when homologous chromosomal segments are accidentally paired in asexual cells such as body cells. Mitotic crossing-over is rare, but it is important in some organismsfor example, some fungi that do not have a sexual cycle use mitotic crossing-over as a source of variation. Furthermore, in the study of human cancer, mitotic crossing-over is thought to be important in allowing recessive cancer-causing mutations to become expressed.

Mitotic crossing-over occurs only in diploid cells such as the body cells of diploid organisms. (In order to study mitotic crossing-over in a haploid organism, diploid cells have to be created artificially.)

Let's look at an example from Drosophila involving the linked genes for body color (y+ = brown, y = yellow) and hair shape (sn+ = straight, sn = singed or wavy). A fly that is a trans dihybrid for these alleles is shown in Figure 5-16. A trans dihybrid should express the dominant phenotypes for both genes in cells all over the body. This indeed will be the case for most cells. However, if a mitotic crossover occurs, this can result in the production of homozygous allele combinations in all heterozygous genes that are located on that chromosome arm distal to the crossover (that is, on the portion of the arm beyond the crossover in a direction away from the centromere). Therefore, recessive alleles will be expressed unexpectedly in the phenotype. Because the cells that are products of a mitotic crossover will produce a group of descendent cells that tend to remain together during development, the recessive phenotypes will be expressed as patches or spots on the body. In the example in Figure 5-16, a twin spot of recessive phenotypes is produced by a crossover between the centromere and the nearest locus, and a single yellow spot is produced by a crossover between the loci (Figure 5-17).

MESSAGE
A mitotic crossover generates homozygosity of alleles of heterozygous loci distal to the crossover.

© 1999 by W. H. Freeman and Company. All rights reserved.