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Epigenetics is the study of heritable changes in a chromosome other than changes in the underlying DNA sequence. The epigenetic inheritance system has been described as ‘soft inheritance’ in comparison to genetics, which is ‘hard inheritance’ (Mayr and Provine, 1980). The inheritance of traits in genetics occurs as a result of rare genetic mutations that involve DNA mutation, but selection is slow in making adaptations to the constantly changing environment. The soft inheritance system of epigenetics, on the other hand, is able to adapt to fluctuations in the environment, such as changes in nutrition, stress and toxins (Wei et al., 2015).

Epigenetics at the cellular level produces cell differentiation by determining the functional types of cell, such as hepatocytes in the liver, neurones in the brain, or skin cells, as well as influencing whether or not they become cancerous. Within the CNS, epigenetics are involved in various neurodegenerative disorders and physiological responses, such as Alzheimer’s disease, depression, schizophrenia, glioma, addiction, Rett syndrome, alcohol dependence, autism, epilepsy, multiple sclerosis and stress. As neurones are incapable of dividing and cannot be replaced after degeneration, epigenetic alterations that cause neuronal dysfunction have to be targeted and modified to prevent chronic kinds of neurodegeneration, which can prove fatal (Adwan and Zawia, 2013).

Epigenetic changes include DNA methylation and histone modification, both of which regulate gene expression without altering the linear sequence of DNA. DNA methylation adds methyl groups to the DNA molecule, which can change the activity of a DNA segment without changing the sequence. DNA methylation typically acts to repress or switch off gene transcription. DNA methylation is implicated in a wide range of processes, including chromosome instability, X-chromosome inactivation, cell differentiation, cancer progression and gene regulation. The flexibility in gene expression is seen early in childhood and can be demonstrated in identical twins, who, even when raised in the ‘same’ environment, can have a different expression of genes. Essentially, DNA methylation is a switch that switches genes in the genotype on or off to produce the phenotype, the human being we actually become, rather than the one determined by a random mix from the gene bank of ‘Mum and Dad’ (Figure 3.6).

Epigenetics can be viewed as a set of bridging processes between the genotype and the creation of the all-important phenotype – a phenomenon that changes the final outcome of a locus or chromosome without changing the underlying DNA sequence (Goldberg et al., 2007). We turn to consider the role of epigenetics in developmental plasticity and the ‘Foetal Origins Hypothesis’, which is concerned with the role of nutrition and malnutrition in healthy foetal development.