The majority of the single deletion mutants of the identified genes in the yeast genome are viable, which suggests redundancy in the some of their function. A way to identify some of the redundancies is to have a query gene deletion mutant that is viable, and crossing this with a large array of viable single deletion mutant of a different mating type. This mating produces haploid double mutants, which can then be used to screen for synthetic lethal double mutants. By using an automated robotic system, synthetic lethal double mutants can be identified in a high-throughput manner. http://www.ncbi.nlm.nih.gov/pubmed/11743205
Tong AHY, et al. (2001) Systematic genetic analysis with ordered arrays of yeast deletion mutants. Science 294(5550):2364-2368.
This is a way to look at the genotypes of the different spores that could result from the mating of two different haploid cells. In nutrient-poor conditions, the newly formed diploid cell undergoes sporulation, where four haploid spores form. Depending on how the recombination occurs, there are four different ways in which the genome can rearrange. Tetrad (T) has a single double mutant spore, which is unable to form a colony. Non-parental ditype (NPD) spores have two types of spores, one of which are parental and the other is recombinant, which could yield two synthetic lethal mutants spores. Parental ditype (PD) have the same genotype as the parental generation, which lacks spores with the double deletion. Sorting the different types of haploid spores allows us to visualize viability of the organism after creating a double deletion.