15.6: Interacting traits- synthetic lethality and co-dominance

The physical linkage of genetic loci is only one of the ways that genes interact (↑), others involve interactions between gene products and the biological processes they mediate. There are interactions between proteins encoded and other gene products within the concentrated confines of the cell (considered later). Perhaps the most dramatic type of interaction, from the perspective of phenotype is known as synthetic lethality.^[521] In such a situation, often but not necessarily, carried out with dominant alleles of two distinct genes, both heterozygotes on their own are viable, while the double heterozygote is dead–the combination is lethal (→). Similarly, it can be the case that two recessive alleles, individually viable as homozygotes and non-viable or display a different phenotype in double homozygous individuals. We can detect the presence of synthetic lethality through various crosses in which individuals with specific expected combinations of alleles fail to appear in the progeny of a cross (←). As long as we can identify the phenotypes of expected progeny, and so count their presence in a population, such deviations can be detected using a 𝛘² analysis similar to our approach to identifying linkage.

The presence of synthetic lethality suggests that the two gene products are involved in a common, essential process. Less extreme interaction outcomes are associated with other types of interactions between alleles of different genetic loci; these are recognized because the phenotype produced by the presence of both alleles is different from the phenotype of either allele on its own. This is distinctly different from the behavior of Mendel’s genetic factors, as noted by Weldon and others).

Synthetic phenotypes can arise in a number of ways. A process may depend upon multiple gene products interacting to form a functional complex, necessary to produce a trait. Two, often paralogous, genes may produce functionally similar gene products. If one is mutated so as to produce little or no functional gene product, the product of the second gene may be sufficient, but if both are mutant, not enough of the functional complex is formed. In some cases, alleles of both genes may be recessive, but when present together, they may appear dominant. Such a situation can be generated using various methods, producing what is known as a “sensitized background” that reveals the roles of gene products in molecular and cellular contexts. There are many complexities associate with mutant behaviors.

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