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Experimental tractability is the practical ability to cultivate a system, manipulate the variables of interest, and obtain an interpretable readout with appropriate scientific controls. In Saccharomyces cerevisiae, inexpensive cultivation, accessible haploid and diploid states, and tools for introducing DNA make controlled changes of genotype feasible. A haploid biological strain exposes a changed allele without a second copy at that genetic locus, while crosses and meiosis permit tests of inheritance. ^[ref]

An accessible readout is not necessarily a direct measurement of the mechanism. Growth of a biological colony integrates many cellular functions. Failure to grow could reflect the intended gene deletion, unsuitable conditions, failed genetic transformation, or another mutation. Matching the genetic background and using scientific controls narrows these alternatives. Ease of doing an experiment and strength of its inference are distinct advantages.

A gene deletion removes a chosen gene sequence from a genome. An experimental gene deletion tests the consequences of that removal in a specified genetic background and environment; lack of an observed phenotype does not prove that the gene has no function under other conditions.

The genetic background is the rest of the genotype against which a focal allele or gene deletion is studied. Other genes can influence the resulting phenotype, so comparisons should specify which background is held fixed.

Evidence transfers at a stated level. Sequence alignment can suggest a candidate relationship between genes; functional complementation tests whether a candidate can supply a particular function in a chosen recipient. Neither alone establishes identical native gene expression, identical signal transduction, or identical consequences in every species. The yeast cross-species complementation experiment is especially useful because it tests a conserved function by intervention rather than assuming it from resemblance. ^[ref]

Even successful functional complementation occurs within a specific genetic background, nutrient supply and imposed gene expression. The enzyme may work with the recipient’s available substrates and partner proteins at an experimentally imposed abundance. Establishing the same role in the donor species calls for experiments there; establishing behavior under another condition calls for experiments under that condition. A conserved component can therefore coexist with context-dependent behavior of an entire pathway. This separation makes model organisms powerful: an accessible experiment supplies a precise mechanistic foothold and a better question for the next system.

A biological strain is a maintained variant or lineage within a species. Naming the species alone does not specify a laboratory genotype or its genetic background.