Let’s fast-forward to the end of the 20th century, to meet the rodents that are helping scientists to unravel the complex connections between diet and epigenetics. There are a number of genes that contribute to coat colour in mice, and one of them is named agouti. There are various versions of the agouti gene, that lead to different coat colours by affecting the levels and types of pigment molecules in the fur. The most interesting version of agouti - to epigeneticists at least - is known as 'agouti viable yellow', or Avy for short.

If the Avy gene has little or no methylation, then it is active in all cells, and the mouse is yellow. These yellow mice are also more likely to have health problems, including obesity, diabetes and cancer. But if Avy is highly methylated, it switches off throughout the entire body. This means the mouse is a sooty-brown colour, with no health problems, even though it has exactly the same agouti gene as a yellow mouse.

In between these two extremes, Avy can be methylated to varying degrees, and this affects the activity level of the gene. The result is a beautiful spectrum of mottled mice, in which Avy activity even differs from cell to cell. Genetically identical littermates range in colour across this spectrum, due to epigenetic variations established in the womb. And besides coat colour, this casts light on the effects of diet on methylation.