Gene Conversion. Tetrad Analysis of Intragenic Recombination
About a dozen different names have been suggested for the phenomenon which here and later in the book will be called conversion*: the abun-dance of synonyms clearly reflects the atmosphere of speculation which until recently has enveloped this phenomenon. For various reasons, however, none of the newly introduced terms gained wide acceptance and recognition, and in modern genetic literature the classical name — is still applied to this phenomenon.
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- Nonreciprocal recombination (Mitchell, 1955); the Mitchell effect (Roman, 1956); transmutation (Horowitz, 1957), transreplication (Glass, 1957), intragenic recombination (Stadler, 1959a), defective copying (Prichard, 1960a), reversion (Magni and Von Borstel, 1962), heteroallelic regeneration (Hurst and Fogel, 1964), asymmetrical crossing-over (Westergard, 1964).Google Scholar
- t refers to the tetratype in interallelic crosses, by contrast to T, which is the tetratype in intergenic crosses (1.9).Google Scholar
- Total rejection of frequent switches within the polaron led to contradictions which will not be considered here. For this reason, Stadler (Stadler, 1963; Stadler and Towe, 1963) proposed a modified varien of the polaron hypothesis. This hypothesis is the las desperate attempt to explain conversion on the basis of copying with exchange of templates. The structural features of Stadler’s hypothesis wil be considered in 4.7. From the standpoint of formal genetics it is a compromise between Freese’s hypothesis and the polaron hypothesis. Accepting the concept of the polaron, Stadler postulated the the number of switches which can take place within it is not one (as in the original polaron hypothesis), and not many, as Freese postulated, but a maximum of two. Sometimes there is one switch and the miscopying continues as far as the end of the polaron, while at other times the miscopying is corrected by a second switch following immmediately after the first. The relative frequency of the single and double switches can vary in different systems. Similar ideas have also been developed by Bernstein (1962, 1964).Google Scholar
- For another possible mechanism, see 5.2.Google Scholar
- This statement is valid only with respect to the copy-choice hypothesis. Within the framework of the breakage-reunion mechanism, such modification is impossible (5.2).Google Scholar
- Crosses of close relatives, leading to homozygotization of the strain.Google Scholar
- The term “proximal” means nearer to the centromere. In this case, tryp-4. If the locus is not oriented relative to the centromere, its left end is regarded as proximal.Google Scholar
- Mutant alleles on the same chromosome (ab × a + b +).Google Scholar