The archetypal apple is – no two ways about it – red. There may be yellow apples or green apples in the grocery store too. In some places, you might even find varieties that are striped or mottled with a profusion of hues, like the gorgeous Cox’s Orange Pippin.

But red – or occasionally, pure, sharp Granny Smith green – is the colour of apples in most alphabet books. It’s an interesting detail, because apples were not always so resolutely monochrome.

The ancestors of the modern apple were wild trees growing in what is now Kazakhstan, on the western slope of the mountains which border western China. Today, wild apple trees still grow there, perfuming the air with fallen fruit and feeding the bears that lumber through the forest, although the wild apples’ numbers have shrunk by 90% in the last 50 years thanks to human development and their future is uncertain.

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The fruits range from pale yellow to cherry red and spring green, but red is not generally more prominent than the other colours. (One apple-loving traveller, Beck Lowe, reports that ironically a commercial Kazakh orchard, like orchards everywhere around the world, is growing Red Delicious and Golden Delicious, apples of American origin.)

Apple colour arises from the expression level of certain genes in the skin, scientists have found. David Chagne, a geneticist at Plant and Food Research in New Zealand, explains that sets of enzymes work together to turn certain molecules into pigments called anthocyanins, the same class of substances that give purple sweet potatoes, grapes and plums their colour.

The levels of these enzymes are controlled by a transcription factor – a protein that regulates how much a gene is expressed – called MYB10, such that the more MYB10 there is, the redder the skin will generally be. In fact, one study found that in apples with red stripes, MYB10 levels were higher in the striped portions of the skin.