Epigenetics

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Key Knowledge:
  • Proportionate influences of genetic material, and environmental and epigenetic factors, on phenotypes


The genotype is not the only contributing factor to the determination of phenotype – it can also be influenced environment or epigenetics

  • The proportionate influence of these other factors explains how individuals with the same genotype can have different phenotypes


Environment

Genetic material (i.e. genotype) codes for specific proteins that determine the physical characteristics of an individual (i.e. phenotype)

  • However, environmental factors may directly influence the properties of a particular protein to change the observable characteristic


A real-world example of a how the environment can trigger a change in phenotype may be seen in the flower colour of hydrangeas

  • Certain hydrangeas change colour depending on the pH of the soil (acidic soil = blue flower ; alkaline soil = pink flower)
  • The pH change alters the molecular structure of the flower pigment, which causes a resultant change in phenotype


hydrangeas


Epigenetics

Epigenetics describes the differences in phenotype that are caused by changes to gene expression patterns 

  • Gene expression patterns may be influenced by genotype but are not genetically pre-determined (can be altered by external factors) 


There are a variety of different factors that may contribute to the expression of a particular trait, including:

  • Age – gene expression levels may change over the course of a lifetime (e.g. hair pigmentation and expression = going grey or bald)
  • Diet – certain foods may trigger specific metabolic responses (e.g. appetite suppressants may alter expression of the hormone leptin)
  • Lifestyle – expression patterns may be influenced by levels of activity (e.g. exercise) or exposure to pathogens (e.g. disease onset)
  • Environment – External conditions can activate certain genes (e.g. UV exposure can stimulate melanin production to cause tanning)


Gene expression profiles are changed by making genes more or less accessible to transcriptional machinery – this can be done via:

  • Acetylation of histone proteins (which causes DNA to become less tightly coiled and hence more likely to undergo transcription)
  • Direct methylation of DNA (which functions to impede the binding of transcription factors and hence reduces gene expression levels)