Marker effects and the nature of the recombination event at the his1 locus of Saccharomyces cerevisiae.

Analysis of unselected and selected (prototroph-containing) tetrads, carrying two or three alleles of the his1 locus in yeast, led to the following conclusions: (1) Conversion at this locus is strongly polarized and asymmetrical. (2) In cases where a crossover is separated from a converted marker by an unconverted segment, these effects are parts of the same event. (3) The distribution of the various classes of tetrad with a crossover associated with conversion at his1 shows that the crossover is at the variable end of the event within the gene. (4) Reciprocal intragenic recombination results from restoration correction in crossover events which end between the alleles. Its occurrence is marker dependent, not distance dependent. (5) A deficiency in reciprocal intragenic recombination is interpreted as an allele-specific disparity in the ratio of conversion to restoration. (6) Marker effects are interpreted in terms of reciprocal influence of markers on each other's correction from heteroduplex, and of interference by heterozygosity with the migration of the cross-strand exchange.