Methodology in Basic Genetics

By Walter J. Burdette | Go to book overview

Herschel Roman, Ph.D.


GENIC CONVERSION in FUNGI*

The hypothetical mechanism of genic interaction known as genic conversion was proposed by Winkler in 19301074 as an alternative to the segmental-exchange hypothesis of crossing over964 to account for the recombination of linked markers. Genic conversion was assumed to occur only in the heterozygote and to consist of an event by which a gene converted its allele into a likeness of itself. Thus, in the heterozygote Aa, A is convertible to a and a to A. In contradistinction to crossing over, from which reciprocal recombinants would be expected, genie conversion could give rise to irregular segregation in the four products of meiosis; that is, the diploid Aa could yield, in addition to 2A:2a segregation, 4A:0a, 3A:1a, 1A:3a, and 0A:4a tetrads. The 4:0 and 0:4 segregations would arise if the A allele or a allele were converted prior to chromosomal duplication in the mother meiotic cell, or if both A alleles or both a alleles were converted after the duplication of the chromosomes. The 3:1 and 1:3 segregations would be expected if, after chromosomal duplication, only one of the two A or two a alleles were converted.

Winkler found support for the conversion hypothesis in the exceptions to 2:2 segregation then being reported in fungi and mosses. More recently, Lindegren561, 562 has invoked genic conversion as an explanation of irregular segregation in his extensive investigations with yeast.

The diagnostic difference between genic conversion and crossing over depends thus on the demonstration of nonreciprocal versus reciprocal recombination. In providing a demonstration of nonreciprocal recombination, it is essential that other

____________________
*
Some of the work reported in this paper was supported by funds from the Public Health Service, National Institutes of Health, Grant No. E-328, and by the Initiative 171 Fund of the State of Washington. I would like also to express my indebtedness to Dr. D. C. Hawthorne and Dr. R. K. Mortimer for their helpful contributions.

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