«As a result, geneticists have borrowed from physics the analogy of a "black box" to represent the uncertainties of gene expression in living persons. [...] At least four technical terms have emerged in molecular biology in the effort to identify the uncertainties that limit and modify genetic determinism.
Penetrance refers to the variable degree to which a gene actually affects or dominates the life and behavior (phenotype) of the individual carrying that gene [...]
Epigenesis designates the mechanism that, at a certain stage of development, acts on networks of cells to supply variations from any predictable pattern [...]
Heterosis causes genes to affect each other in unexpected, nonadditive ways [...]
Developmental noise refers to random molecular events within cells during development, events that produce accumulating deviations from the norm [...]
All four terms refer to processes as yet little understood. [...]
All these considerations and black-box factors should mute the promises the HGP [Human Genome Project] tends to make about curing our individual and social ills. »
-- from chapter 6 of Roger Shattuck's book Forbidden Knowledge