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Abstract: The existence of a normal sexual process in Agaricus bisporus (LANGE) IMBACH, known as secondary homothallism controlled by alternate alleles of a mating type gene (bipolar incompatibility), has been established in previous studies (MILLER, 1972; RAPER et al., 1972). Cytological work by COLSON (1935) and EVANS (1959) and the analysis of monosporous progeny from specific basidial types by PELHAM (1967), ELLIOTT (1972), and MILLER (1971) has provided information on the relationship of nuclear behavior to the expression of phenotypic characters. Definitive evidence for recombination and segregation was provided by the genetic studies of RAPER et al. (1972) with natural and induced auxotrophs (nutritionally deficient mutants). All phenotypic classes expected for independent assortment and recombination of alternate alleles of two nutritional genes were recovered in the progeny of a cross between two homokaryotic strains. Understanding the basic life cycle of A. bisporus is a prerequisite to breeding and selecting for a desirable recombination of characters. The use of auxotrophic mutants as parents in a cross was necessary in these initial breeding studies so that nuclear exchange could be confirmed and subsequent developments traced. Easily recognizable phenotypic characters, "wild type" morphology (hence forth referred to as W morphology to avoid confusion with the classical concept of wild type) and color were selected for the study. MILLER (1971) reported segregation for morphology with a single dominant gene responsible for normal (N) morphology. Preliminary results indicated inheritance of color to be more complex.
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