Dissection of Genetic Pathways in C. elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 140

A. Determining Whether Two Genes Function in the Same Pathway
Following the recovery of mutations based upon the strategies discussed above,
great effort should be put to carefully characterize the phenotypes and genetic nature
of mutations (i.e., null, hypomorphic, or hypermorphic). This information is essen-
tial for genetic interaction analysis in which the defects of double mutants are
compared to those of single mutants, allowing one to determine whether mutated
genes act in the same or distinct pathways that regulate a particular biological
process. It is particularly important for this analysis to use null alleles or a null allele
and a strong-loss-of-function allele with no activity in the assayed biological process
for double mutant construction. Double mutants with a phenotypic severity similar
to that of the more severe single mutant suggest that the two genes work together or in
series within the same pathway ( Fig. 8 i). Lacking a genetic interaction, the pheno-
type of the double mutant would be expected to be equal to the additive defects of the
single mutants ( Fig. 8 ii). Double mutants displaying a more severe phenotype than
the expected combined loss of each indicate a synergistic (or synthetic) interaction
( Fig. 8 iii) ( Boone et al., 2007; Guarente, 1993; Mani et al., 2008 ). The assumption in
this case is that the two genes likely act in parallel pathways that converge on a
common function or activity. These interpretations do not apply for genetic inter-
actions between two hypomorphic alleles. Because of their residual activity, the
linearity of the pathway where they reside is not completely interrupted, any scenario
above can be caused by two hypomorphic alleles that either function in the same
pathway or the distinct pathways. Other functional information is often needed to
Fig. 8 Implications of genetic interaction between two null alleles or a null allele and a strong-loss-of-
function allele with no activity in the assayed biological process. To determine whether two alleles (A and
B) function in the same pathway, the double mutant (AB) homozygous for both alleles is constructed.
Comparing the severity of this double mutant with that of single mutants can provide insight into their
functional relationship. (i) If the phenotypic severity of the double mutant is similar to that of the single
mutant with the more severe phenotype, it implicates that the two genes act in the same pathway; (ii) if
the phenotypic severity of the double mutant is the sum of that of the single mutants, it implicates that
these two genes have no genetic interaction; (iii) if the phenotypic severity of the double mutant is more
than the sum of that of the single mutants, it indicates that the two genes act in parallel pathways that
converge on a common function. (For color version of this figure, the reader is referred to the web version
of this topic.)