Epistasis is the concept discussed. One has a reasonable empirical understanding of point mutations. Point mutations are however not independent as simple linear thinking would suggest. This gives rise to epistasis.
Two mutations with qualitatively similar effects can produce a mutation with an opposite effect. Poorly understood interactions between mutations exist and give rise to the epistasis. One might call these interactions non-linear in a lack of a better word. The proposal that has been developed is global epistasis suggesting that genes and even large units would tend to have like coherent units.
My own intuitive view of DNA is based on quantum coherence in DNA length scales predicted by the TGD based view of chemical DNA as a chemical "shadow" of what I call dark DNA.
Dark DNA is realized as sequences of dark protons at the monopole flux tubes of the magnetic body associated with the ordinary DNA. It relies on a universal realization of the genetic code based on a completely unique icosa-tetrahedral tessellation of hyperbolic 3-space (light-cone proper time constant 3-surface in Minkowski space M^4). Genetic code might be universal at the level of the magnetic body and biological realization(s!?) would be only of the many. This would make the Universe intelligent, conscious and evolving in all scales using the fundamental binary coded with a codon as a 6-qubit unit (see this).
Not only codons but also genes would be quantum coherent units interacting like particles. For instance, dark genes consisting of N dark codons (each with 3 dark protons) would emit 3N-photon as a single unit in communications based on 3N-resonance, which implies that identical dark genes can communicate with each and that the modulation of frequency scale as a message is coded to a sequence of resonance peaks analogous to a sequence nerve pulses. This is a quantum generalization of what occurs in radio communications. Even larger quantum coherent units can be considered.
This implies that mutations are not anymore independent as in the picture based on chemistry alone. Mutations could have profound effects on the communications by 3N-resonance and 3N frequency resonance is not anymore complete if one codon changes. Therefore the effects of two or more mutations on dark gene communications do not simply add up. This raises the hope that their interactions might be understood some day.
For a summary of earlier postings see Latest progress in TGD.
For the lists of articles (most of them published in journals founded by Huping Hu) and books about TGD see this.