It's all relative.

It started as party chatter.

My hands are different sizes, not so much that most people can tell, but I can. The real difference is the thumbs. The distal joint of my right thumb, but only the right one, bends back 90°, 'hitchhiker's thumb'. The medical term is distal hyperextensibility. It's an autosomal recessive trait, so theoretically you can't have just one. At parties, I said, “I have someone else's hand. When they were putting me together, somebody dropped a hand and stepped on it, so they had to go for spare parts.” Great conversation starter, for a while.

Then I read about the Chimaera Project.

A chimaera is an organism composed of two genetically distinct cell lines. Natural chimaeras occur when fraternal twin embryos fuse early enough to develop into a single entity, with no extra limbs or organs.

Credit: JACEY

I saw a TV show once where a woman sought legal custody of two children on the grounds that she was their biological mother. The problem was, DNA tests kept showing that she wasn't. Finally, her doctor figured out she was a chimaera. One of her genomes had been tested; the other one was the children's mother. The doctor told the woman's lawyer this was a rare condition, but he asked how she knew that, as no one ever tested for it. I was living in an urban area with several medical research institutions. Anonymous donors funded one of them to find out how common chimaeras really were. Maybe the donors saw the same TV show.

The clinic where the Chimaera Project was based was nearby, and I managed to get an appointment. Dr Richard Mott was intrigued enough by my thumbs to take some samples from them. He said my results would be ready in a few weeks, but three days later, his assistant called and asked me to come in. When Dr Mott asked to see me personally, I thought he just wanted more samples. Instead, he had my results.

“You were an easy case, Mr Treadwell. Your left hand is female.”

When I was bemused instead of horrified, he told me that opposite-gender chimaerism goes unnoticed unless genitals or hormone-secreting organs are involved. When he remarked on how calmly I took the news, I assumed he had other patients who took it badly. He still wanted me to see a counsellor.

Dr Mary Austin was pleasant enough, and she really did seem concerned about me. Towards the end of the session, she asked if I felt like two people.

“I'm the same person I've always been, Dr Austin. I never bought the unique genome story, even when the anticyclones came.”

Anticyclone is a meteorological term, co-opted to mean someone who opposed cytogenetic cloning — anti-cy-clone. A lame coinage, certainly, but the bloggers immortalized it. I could tell she hadn't expected me to know about that, but not whether she was pleased or upset.

“Two people? Does anyone actually believe that?”

“You'd be surprised. You actually remember the anticyclones?”

“Barely. I was a child, but I remember the mobs, and the bodies in the streets. And the older kids told stories.” The older kids told of surgical scars ripped open to recover cloned organs, not because they were worth any money, but to give them decent burials. I thought they were just making it up to scare us, as older kids do, but I have since read the histories. Know-nothings insisted for decades that a zygote was a person because it had a unique genome; then they discovered identical twins. Thus was the anticyclone movement born. Because the cloned cell could theoretically grow into the patient's twin instead of an organ, they insisted the organ was a separate person, too.

“Why would I be surprised?”

“Maybe you wouldn't be, Mr Treadwell, as you remember the anticyclones,” was all she said. At least I had some better party chatter to go with my thumbs.

The Chimaera Project must have been a scientific success because before too many more parties, I found out it had generated a backlash. It seemed there were quite a few of us, and we had attracted the wrong sort of attention. Some people thought we were abominations.

I liked to think the anticyclone movement withered because the public finally saw the value of all those people who lived because they could regenerate a damaged organ, rather than died. Cynics said the movement perished of intellectual bankruptcy. It didn't matter. Like the mythical hydra that grew two heads where one had been lopped off, this beast would not be slain so easily.

The evening news said Dr Mott's clinic was burning. Dr Austin's husband told the police she never came home from work yesterday. Her office had been ransacked, and her patient files were missing. The brick that came through my bedroom window later that week was wrapped in a piece of paper that said: “And if thy hand offend thee, cut it off: it is better for thee to enter into life maimed, than having two hands to go into hell, into the fire that never shall be quenched.”

The police treated my incident as the work of a lone vandal or maybe a prank, certainly not the tip of an iceberg decades deep. The older kids of my childhood told another scary truth: there is no place to hide from the madness. So, I shall fight it where I can and crush it where it rises. My one-time sister's hand will strike the blows. The party is over.

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Cash, G. Thumbs. Nature 475, 260 (2011).

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