The language of genes has become common parlance. We know they make your eyes blue, your hair curly, or your nose straight. The media tells us that our genes control the risk of cancer, heart disease, alcoholism, or Alzheimer's. The cost of DNA sequencing has plummeted from billions of dollars to a few hundred, and gene-based advances in medicine hold huge promise.

So we've all heard of genes, but how do they actually work? According to legend, Ernest Hemingway was once given a six-toed cat by an old sea captain, and her distinctive descendants still roam the writer's Florida estate today. Scientists now know that the fault driving this profusion of digits lies in a tiny genetic control switch, miles away (in molecular terms) from the gene that "makes" toes. And it's the same mistake that gives rise to multi-toed humans too. There are 2.2 meters of DNA inside every one of your cells, encoding roughly 20,000 genes. These are the "recipes" that tell our cells how to make the building blocks of life, along with myriad control switches ensuring they're turned on and off at the right time and in the right place. But rather than a static string of genetic code, this is a dynamic, writhing biological library. And figuring out how it all works―how your genes make you, you―is a major challenge for researchers around the world. Drawing on stories ranging from six-toed cats and stickleback hips to wobbly worms and zombie genes, geneticist Kat Arney explores the how our genes work, creating a companion to the book of life itself.