Another set of "unknowns" is HA2, HB2 and a diploid made by crossing HA2xHB2. The mating mixture produced by mixing HA2 and HB2 should be purified by streaking for single colonies on YED. (See Laboratory Methods) If the mating conditions are poor the single colonies produced may be a mixture of diploids and unmated haploids.
Yeast strains usually come from the supplier growing on agar slants. Contamination may be a problem when students use the master set of yeast strain slants as the source of their strains. You may wish to subculture the strains on YED plates. One subculture plate containing the tester strains and unknowns will supply enough yeast for a class. If several groups need access to the yeast at the same time, you may want to make several subculture plates (See segment on Subculturing Yeast on video tape III).
You may wish to have the students make a paper grid to tape on the bottom of the plate or they could draw a grid directly on the bottom of the plate with a marking pen.
Stress that a new sterile toothpick should be used for each different strain and mixture.
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Last updated Friday July 11 1997