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| Modern Genetic Analysis Chapter 7. Gene Mutations Mutational Analysis Figure 7-40. Testing strains of Neurospora crassa for auxotrophy and prototrophy. In this experiment, the test utilizes 20 progeny from a cross of an adenine-requiring auxotroph and a leucine-requiring auxotroph. Genotypically, the cross is ad. leu+ × ad+. leu, and the progeny can carry any of the four possible combinations of these alleles. To test the progeny, the geneticist attempts to grow cells on various kinds of gelled media in petri dishes. The media are minimal medium (Min) with either adenine (Ad, top left), leucine (Leu, top right), neither (bottom left), or both (bottom right). Growth appears as a small circular colony (white in the photograph). Any culture growing on minimal medium must be ad+ . leu+, one growing on adenine and no leucine must be leu+, and one growing on leucine and no adenine must be ad+. All should grow on adenine plus leucine; it is a kind of control to check viability. As examples, culture 8 must be ad . leu+, 9 must be ad . leu, 10 must be ad+ . leu+, and 13 must be ad+ . leu. (Anthony Griffiths)

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