Environmental factors that can influence an organism's phenotype includes:

• Temperature

• pH level

• Food availability

• Light exposure

• Wind exposure

For example, the hydrangea flower is affected by external environmental factors. The range of colours in the flowers (blues and purples) is directly linked to the pH level in the soil, along with the ratio of additives (such as aluminium ions). More acidic soil is conductive to blue flowers. Internal environmental factors include the actions of hormones, such as the release of GnRH (gonadotropin-releasing hormone), which triggers the start of puberty. Epigenetics is the study of inheritable (yet reversible) change in gene expression without a change in the DNA sequence. In relation to the human genome, the epigenetic factors are 'above' the DNA and exert control over it through the activation and deactivation of genes, which hence influences phenotype by controlling gene expression. Environmental factors can contribute to the addition or subtraction of epigenetic chemical factors and turn certain genes on or off, for example, diet and stress can affect chromatin structure and gene expression. Epigenetics affects gene expression, and therefore it can affect the growth and development of organisms. A key example of where epigenetics is seen is in twins. Despite having identical genomes, they have different epigenomes based on their exposure to different environmental factors. For example, one twin might have smoked cigarettes while the other one doesn't, and in the twin that did smoke, this changes their epigenome and increases their risk of developing their lung cancer.