Excerpt from The Family Tree by Carole Cadwalladr, plus links to reviews, author biography & more

There are people who spend their whole lives looking for genes. They're the big-game hunters of our times, although they use microscopes rather than double-barreled shotguns. Alistair has a tendency to sneer at them, but I suspect that's a professional thing.

How to Find a Gene for a Trait
1. Take a fruit fly.
2. Expose it to an X ray.
3. Mate it with another fly.
4. Study the defects of the progeny.
5. Isolate the mutated gene.

He's described the process for me. From the mutations of the offspring, you have to track back to find the mutated gene. The child is father to the man. You hunt for the gene that's been changed, distorted, knocked out.

We were in a park at the time. And he tried to illustrate the idea by pulling petals out of a daisy.

"Say that is a gene you've knocked out." He pulled out a cluster of petals to leave a hole.

"I know that game," I said. "‘He loves me, he loves me not.' Lucy and I used to play it when we were children."

He frowned at me and placed another daisy over the top of the first. "And pretend this is the progeny of the daisy with a hole." He began to pull out petals at random. I was trying hard to follow.

"Can you see?"

"I think so," I said.

"You see this other flower, the ‘child,' has a mutation. But you can work out what's missing, what should be there, from the ‘parent.' It's not really like that, but do you get the idea?"

I nodded although I didn't, not really.

"It's not a very good example," he said. "But you have to work in reverse. It's the science of the missing gap. You can see a thing clearly only by the shape it leaves behind. From the effect that is produced by it not being there."

"Like my mother?"

I watched him pulling petals out of the daisy and waited for a reply.

"That's different," he said eventually. I caught the edge of exasperation in his voice. "It's science, not emotion."

I shrugged my shoulders and watched him pull out the last remaining petal. He loves me not. Although possibly I'd miscounted.

From The Family Tree by Carole Cadwalladr, pages 1-17. All rights reserved. No part of this book maybe reproduced without written permission from the publisher.