Deterministic versus random factors

The characteristics or traits that define a biological system can be either inherited, or modified or acquired/learned during the lifetime of the system.

While inherited traits are mostly deterministic in that the information is transmitted from the parents via the genome, the DNA can be damaged and errors can occur in the replication processes, leading to random modifications or mutations. Modifications also occur in response to external factors. For example, in single-celled organisms, regulatory networks respond to the external environment, optimising the cell at a given time for survival in this environment. Thus a yeast cell, finding itself in a sugar solution, will turn on genes to make enzymes that process the sugar to alcohol. The same principle is also allpied in multicellular animals, for instance, to modify the gene cascades that control body shape during development. In extreme cases, modifications of the finely balanced regulatory and metabolic netowrks that exist in normal cells, can lead to complex diseases, such as cancers.

When the modifications occur at the level of DNA, the changes can also be inherited by the children of the system. Examples of such 'epigenetic' modifications include DNA methylation and histone deacetylation, both of which serve to suppress gene expression without altering the sequence of the silenced genes.