A culture comprising at least one set of the chromosomes of the Agaricus bisporus line B12998-s39, the culture of the line B12998-s39 having been deposited under the NRRL Accession Number 50899, wherein said chromosomes comprise all of the alleles of the line B12998-s39 at the sequence-characterized marker loci listed in Table I. Methods are provided for introducing a desired trait into a culture of Agaricus bisporus line B12998-s39, as well as methods and processes for producing hybrid mushroom cultures. A method of mushroom strain development is further provided.