of H53 and H98 were included for each condition as a
negative control for mating. Plating on Hv-Ca medium
without uracil was also included in the experiment to
control for spontaneous reversion to uracil prototrophy
(although this was not likely as the pyrE2 gene is not mutated
in H53/H98, but is completely deleted) or the presence
of any ura + strain; no colonies were observed for any
of these conditions. The transfer frequency was calculated
by dividing the average number of colony-forming units
(CFU) per ml on selective medium (Hv-Ca with uracil) for
each replicate set by the corresponding averaged total CFU
count (CFU/ml on Hv-YPC + thy medium).