(a) Wild-type Neurospora fungus are able to grow on minimal medium (a carbon source and inorganic salts only). Individual conidia (asexual, haploid spores) are exposed to X-rays, which are known to be mutagenic. The mutagenized spores are grown on complete medium (minimal medium plus a full range of amino acids), and are crossed with non-mutagenized Neurospora. The diploid offspring then produce haploid ascospores, which are dissected and transferred individually to new tubes with complete medium.

    (b) The haploid spores grow asexually as haploid fungi on complete medium.

    (c) Spores from each of the haploid fungal cultures are tested for ability to grow on minimal medium: most grow, some do not. Inability to grow (culture X) indicates that a genetic mutation affecting ability to produce some critical growth substance was induced in the parent spore by the X-rays.

    (d) Returning to the cultures in part (b) that showed a "no growth" phenotype in part (c), individual haploid spores are again grown on minimal and complete media as controls, and on a series of minimal media each supplemented with one amino acid.

    (e) Spores from each culture grow on one and only one of the supplemented media. In the example, growth on minimal medium + arginine indicates that the genetic mutation altered the ability of the particular fungal culture to synthesize arginine. The strain will therefore be designated arg^- [read, 'arg minus'] to indicate a defect of arginine synthesis.

    Beadle and Tatum repeated this experiment for hundreds of mutagenized Neurospora, and obtained scores of nutritional mutants that were unable to grow on minimal media, without the addition of particular amino acids.

Homework: The test in (d) includes minimal and complete media growth experiments as "controls": what do these experiments "control" for? Hint: supposing growth occurred on both minimal and complete medium, what would you conclude?