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Modern Genetic Analysis Chapter 5. Recombination of Genes
Summary

Recombination is a process that takes two parental haploid input genotypes (such as the two gametes that form a diploid organism) and creates haploid output genotypes (such as gametes) that have new combinations of the input alleles. In eukaryotes, most recombination takes place at meiosis. There are two meiotic processes that generate recombinants: inde-pendent assortment and crossing-over.

Independent assortment shuffles allele combinations by the equal segregation of heterozygous alleles inde-pendently on separate chromosome pairs. Independent assortment produces a recombinant frequency of 50 percent.

Crossing-over is a breakage and reunion process that occurs between nonsister chromatids. In a dihy-brid for linked genes, a meiosis in which there is a crossover between the loci will produce two reciprocal recombinant products of meiosis. There are two types of dihybrids for linked genes, cis (A B/a b) and trans (A b/a B). In the case of linked heterozygous loci the recombinant frequency is between 0 percent and 50 percent, but the precise value, a constant for those specific genes, depends on the physical distance between the two genes on the chromosome. The frequency of recombinants (RF) is used as a measure of map distance: 1 percent RF = 1 map unit. This technique can be used to draw maps of the relative positions of gene loci on a chromosome. Map distances are generally additive. The most convenient crosses for deducing RF are testcrosses of dihybrids or multihybrids. Close to the 50 percent limit for recombinant frequency, it is necessary to use a statistical test called the chi-square test, which generally tests a null hypothesis and, in the case of linkage analysis, tests whether or not two loci are segregating independently.

Crossing-over can also occur rarely at mitosis. Mitotic crossing-over can result in homozygosity of heterozygous loci distal to the crossover.

Crossing-over occurs by the formation of a DNA molecule that is a heteroduplex of strands from each parental chromatid. In fungi, if this heteroduplex is not repaired, nonidentical sister spores are found in octads. The extent of heteroduplex formation and the presence or absence of repair can explain the range of aberrant allele ratios found in fungal asci.

If appropriate selective methods are available, the frequency of intragenic recombinants can be used to map mutant sites within a gene.

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