genetics versus epigenetics

Both genetics and epigenetics are branches of biology concerned with the study of how cells and their host organisms store, regulate and pass on information to the next generation. Where they differ is in the nature of information they store and the persistence of that information. Let's unpack these concepts. 

genetics: information about how to make an enzyme
epigenetics: information about when to make an enzyme, ie gene expression



Genetics is concerned with genes. From a structural perspective, genes are molecular sequences composed of 4 chemical elements frequently referred to as 'bases' or 'nucleotides' or even 'letters' - A, C, G, T.  These long sequences exist in pairs that wrap around each other in a double helix - the famous DNA molecule.  

Humans have approximately 20,000 genes, each of which encode one or more functional elements in cells and influence physical traits (phenotypes).  

From an informational perspective, each gene contains in its sequence the instructions for making one or more related functional elements in cells. Everything that a cell requires - every component - is precisely defined by a corresponding segment of DNA or gene. 

The preservation of the sequence of letters in DNA is extremely important. Consider that each time a cell divides, it faithfully and flawlessly reproduces the identical sequence, and in an adult human (30-year-old average man, who is 1.72 m tall and weighs 70 kg) who originated from a single celled embryo, there are approximately 37 trillion cells [23829164]. 

Other than accidental random damage (due to radiation or oxidants), cells will only permit changes to DNA under very specific and controlled circumstances such as the recombination that occurs during the production of male sperm and female eggs. In fact the structure of DNA is so resilient, that scientists have been able to recover one million years old DNA from archaeological sites [33597750].  

A useful metaphor is to think of DNA as an enormous encyclopaedia with instructions and blueprints for making every component in any type of cell under any condition. An identical copy of this encyclopaedia is stored in each cell. Depending on the cell type and its particular needs, a different section of the encyclopaedia is utilised.



A neuron that specialises in transmission of electrical pulses will depend on genetic instructions from a different part of the encyclopaedia compared to a muscle cell whose function is to build and maintain contractile fibres. Depending on the cell and host age, a neuron will change how much or how little of the genetic electrical transmission genes it uses. This is where epigenetics comes in. 

Each cell type arranges its DNA in such a way that only the relevant and frequently used parts are easily accessible. It does this by annotating sections of DNA with various molecular marks. One of these marks is known as methylation and involves marking a 'C' letter in DNA with a methyl group (ie we say it becomes methylated). If a gene has a lot of 'C' letters methylated the cell uses a lot less of this gene or sometimes none at all. The study of how DNA function is altered by such marks is known as epigenetics with epi meaning "above".

Epigenetic modifications in DNA are essential to giving each cell its 'character'.  Is it a neuron or a muscle cell? Is it a young neuron or an old neuron? Is it a neuron whose host is on a ketogenic diet?  

Whilst DNA encodes all aspects of how to make things in the cell, the mechanisms associated with epigenetics govern if and when things are to be made. Each cell is constantly surveilling it's environment to ensure it can respond to changes in the availability of nutrients, the presence of hormones and many other environmental factors. When it perceives a change that is long lasting, say a change in diet, it alters the way it accesses its genes. It does this by using epigenetic marks on segments of DNA, altering emphasising the expression of some genes whilst de-emphasizing or even shutting down the expression of others. 

The Dutch Winter Hunger: during 1944 the Nazis blocked food supplies to the Netherlands, plunging much of the country into famine until the allies liberated the Danish in 1945. This tragic period of famine, when more than 20,000 people died of starvation, set the stage for a remarkable experiment. Women who were pregnant during this time gave birth to a cohort of children who would develop common metabolic disturbances: higher rate of obesity and type 2 diabetes. Scientists discovered that during the period of starvation the foetal genomes became epigenetically reprogrammed to anticipate conditions of nutritional scarcity. Under such conditions, food binging and fat retention are beneficial. When the anticipated nutritional scarcity did not occur that behaviour was maladaptive and led to metabolic disturbances [21802226]. 

Extending our metaphor, if genetics is represented by an encyclopaedia, then epigenetics represents whether the books of that encyclopedia are laying open for reading or stored on shelves - as well as the compilation of bookmarks, sticky notes and highlighted text amongst their pages. Epigenetics is not only associated with determining which parts of the encyclopaedia are to be used, but also contain a recent history of personal and parental environmental experiences.

The relationship between genotype and phenotype is intermediated by epigenetic marks on DNA (the epigenotype), i.e. : 

genotype -> epigenotype -> phenotype


Genetic determinism

Much has been said about epigenetics as being the answer to genetic determinism, ie that it offers an escape from our genetic destiny. This is not strictly true. Genetics defines the boundaries of what is possible whilst epigenetics determines where within that scope genes will be expressed. In some cases that scope is very narrow: if you carry the gene for blue eyes, there is no diet or exercise regime or drug that will change eye color.