Identifying mutations in a yeast strain > Cells of a triple-mutant yeast strain (leu- his- trp-) were spread either on minimal medium or on minimal medium supplemented with various combinations of histidine, leucine, or tryptophan. The cultures were grown at either 25C or 37C for 3 days. Colony numbers in each plate were counted, and the data are listed in the following table. > > !enter image description here > > What kind of mutation most probably causes the his- phenotype?What kind of mutation most probably causes the leu- phenotype? > > A.Point mutation > > B.missense mutation > > c.nonsense mutation > > d.conditional mutation > > e.deletion mutation **My questions** : What is the meaning of confluent in this context ? Can anyone give me a hint to solve this question ? I have no clue how to solve this.

For starters, you can just google "confluent". It means that there are no colonies, but that the cells cover the whole plate.

So when you provide all three amino acids, the cells fill up the whole plate. If you leave His out, but keep the plate at 25 degrees, the plate still fills up with cells, so the His- allele isn't hurting the cells at all at 25 degrees, but it's pretty nasty at 37 degrees. That makes it conditional; the mutation only impairs His metabolism at some temperatures.

There is not a single cell which is surviving a lack of Leu, but there are some which are surviving the lack of Trp, and the lack of His at harmful temperatures. Presumably, those are the small number which are lucky enough to acquire mutations which compensate for the original deleterious alleles, but NO cells were able to overcome the problem with the Leucine metabolism. This suggests that the Leucine mutation might be a total deletion.