Ratio

FIGURE 14.27 Gene Conversion Following Crossing Over

Crossing over between two DNA molecules creates a short region of heteroduplex, very close to the Holliday junction (see Fig. 14.04). Sometimes, as shown here, this heteroduplex lies within a gene with two different alleles. A) Two DNA molecules that are about to cross over each carry different alleles (r or R) of the same gene. Crossing over occurs within the gene and a short region of heteroduplex that contains one strand of allele r and the other strand of allele R is generated in one of the daughter DNA molecules. B) Gene conversion occurs when the mismatch repair system corrects the heteroduplex, in this case giving a molecule with the r allele in both strands. C) Gene conversion perturbs the final ratio of the two alleles, as seen following cell division. If no heteroduplex were formed between r and R, a ratio of 2r: 2R would be obtained (not shown). If the heteroduplex is formed, but gene conversion does not occur, a 3r: 1R ratio results, whereas heteroduplex formation followed by gene conversion gives 4r: 0R.

be preserved in a large number of progeny. Gene conversion is thought to occur in all or most organisms, but is only detectable under special circumstances. In practice, it is seen most easily in fungi of the Ascomycete group (yeasts, Neurospora, etc.) because of their developmental pattern. Sexual reproduction results in the formation of a zygote from the fusion of egg and sperm. Meiosis in these fungi then produces a cluster ascomycete Type of fungus that produces four (or sometimes eight) spores in a structure known as an ascus

Gametes (n)

Fertilization

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