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  Section: Genetics » Tetrad Analysis, Mitotic Recombination and Gene Conversion
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Gene conversion

Tetrad Analysis, Mitotic Recombination and Gene Conversion in Haploid Organisms (Fungi and Single Celled Algae)
Tetrad analysis 
Analysis of ordered tetrads
Analysis of unordered tetrads
Mitotic recombination and parasexual cycle in Aspergillus 
Gene conversion.
Since recombination is a reciprocal event, segregation in a heterozygote should yield tetrads with 2 : 2 ratio (2A : 2a) as described earlier in this section. In case of yeast and Neurospora, departure from these expected results were obtained. As a hypothetical example, when a+b/a+b was crossed with ab+/ab+, F1 was a+b/ab+. In a very' small frequency (4/1000), the recombinant asci, a+b+, were available, but their reciprocal recombinant ab was not available. Such results could not be explained on the basis of mutations, since the frequency of such events was much higher than expected on the basis of spontaneous mutations, and the event is always directional (in heterozygote a1/+, a1 → +, and never a1 a2). The unusual events were termed gene conversion, a term which was earlier used by Winkler (1930) for an apparent conversion of one gene into another.

During tetrad analysis also, unusual ratios like 6 : 2, 2 : 6, 5 : 3, 3 : 5, etc. were obtained. In these examples, it appears, as if some genes (or alleles) in the cross got converted into the opposite type, without involving any reciprocal exchange event. If gene conversion is- a post-meiotic phenomenon, one can assume that 6 : 2 or 2 : 6 ratio will be produced due to conversion of a whole chromatid (both strands of DNA get converted); this phenomena is described as chromatid conversion, as against half chromatid conversion for 5 : 3 or 3 : 5 ratio (only one of the two strands of DNA gets converted).

Some of the properties of gene conversion include the following : (i) There is always a gradient or polarity of conversion frequencies along the gene, if different alleles in a micromap of the gene are considered. In other words, sites closer to one specific end of the micromap show higher frequencies than do those farther away, (ii) Gene conversion of an allele is often (50%) accompanied with crossing over in the flanking regions; this exchange always occurs nearest the" allele undergoing gene conversion (Fig. 11.18; conversion of m1 involves crossing over in region I and that of m2 involves crossing over in region II); further the crossing over involves invariably the chromatid involved in gene conversion,
A cross a+m+1b+ x am+2b, showing association of gene conversion with crossing over in flanking region; (a) chromatids of a zygote, (b) genetic constitution of a tetrad (conversion m1 à + is accompanied by a crossover between a and m1 i.e. region I).
Fig. 11.18. A cross a+m+1b+ x am+2b, showing association of gene conversion with crossing over in flanking region; (a) chromatids of a zygote, (b) genetic constitution of a tetrad (conversion m1 + is accompanied by a crossover between a and m1 i.e. region I).

(iii) A single conversion event may sometimes involve several alleles of a gene, if the cross is heteroallelic at several sites. This phenomenon is called co-conversion, and its frequency increases with a decrease of distance between alleles undergoing co-conversion.

In view of the above discussion on gene conversion, it is obvious that the phenomenon of recombination includes following two type of events : (i) crossing over, which is a reciprocal exchange of segments and (ii) gene conversion, which is an event involving non-reciprocal transfer of information. Since crossing over and gene conversion differ from each other, we need to find out a possible molecular mechanism of recombination, which should explain both these phenomena. The copy choice theory, earlier described is inadequate for this purpose for reasons described earlier. But several hybrid DNA models have been proposed which explain both crossing over and gene conversion. A brief account of hybrid DNA models was included in Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes) and a more detailed discussion is presented in Molecular Mechanisms of Recombination.

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