the Third Twin (1996)

“Good point. Your DNA is very similar to mine and everyone else’s in the world. We even have a lot in common with the animals, because they’re made of the same proteins as we are.”

“So how do you tell the difference between Dennis’s DNA and mine?”

“Between the words there are bits that don’t mean anything, they’re just gibberish. They’re like spaces in a sentence. They’re called oligonucleotides, but everyone calls them oligos. In the space between ‘five’ and ‘toes,’ there might be an oligo that reads TATAGAGACCCC, repeated.”

“Everyone has TATAGAGACCCC?”

“Yes, but the number of repeats varies. Where you have thirty-one TATAGAGACCCC oligos between ‘five’ and ‘toes,’ I might have two hundred and eighty-seven. It doesn’t matter how many you have, because the oligo doesn’t mean anything.”

“How do you compare my oligos with Dennis’s?”

She showed him a rectangular plate about the size and shape of a book. “We cover this plate with a gel, make slots all across the top, and drop samples of your DNA and Dennis’s into the slots. Then we put the plate in here.” On the bench was a small glass tank. “We pass an electric current through the gel for a couple of hours. This causes the fragments of DNA to ooze through the gel in straight lines. But small fragments move faster than big ones. So your fragment, with thirty-one oligos, will finish up ahead of mine with two hundred and eighty-seven.”

“How can you see how far they’ve moved?”

“We use chemicals called probes. They attach themselves to specific oligos. Suppose we have an oligo that attracts TATAGAGACCCC.” She showed him a piece of rag like a dishcloth. “We take a nylon membrane soaked in a probe solution and lay it on the gel so it blots up the fragments. Probes are also luminous, so they’ll mark a photographic film.” She looked in another tank. “I see Lisa has already laid the nylon on the film.” She peered down at it. “I think the pattern has been formed. All we need to do is fix the film.”

Steve tried to see the image on the film as she washed it in a bowl of some chemical, then rinsed it under a tap. His history was written on that page. But all he could see was a ladderlike pattern on the clear plastic. Finally she shook it dry then pegged it in front of a light box.

Steve peered at it. The film was streaked, from top to bottom, with straight lines, about a quarter of an inch wide, like gray tracks. The tracks were numbered along the bottom of the film, one to eighteen. Within the tracks were neat black marks like hyphens. It meant nothing to him.

Jeannie said: “The black marks show you how far along the tracks your fragments traveled.”

“But there are two black marks in each track.”

“That’s because you have two strands of DNA, one from your father and one from your mother.”

“Of course. The double helix.”

“Right. And your parents had different oligos.” She consuited a sheet of notes, then looked up. “Are you sure you’re ready for this—one way or the other?”

“Sure.”

“Okay.” She looked down again. “Track three is your blood.”

There were two marks about an inch apart, halfway down the film.

“Track four is a control. It’s probably my blood, or Lisa’s. The marks should be in a completely different position.”

“They are.” The two marks were very close together, right at the bottom of the film near the numbers.

“Track five is Dennis Pinker. Are the marks in the same position as yours, or different?”

“The same,” Steve said. “They match exactly.”

She looked at him. “Steve,” she said, “you’re twins.”

He did not want to believe it. “Is there any chance of a mistake?”

“Sure,” she said. “There’s a one-in-a-hundred chance that two unrelated individuals could have a fragment the same on both maternal and paternal DNA. We normally test four different fragments, using different oligos and different probes. That reduces the chance of a mistake to one in a hundred million. Lisa will do three more; they take half a day each. But I know what they’re going to say. And so do you, don’t you?”

“I guess I do.” Steve sighed. “I’d better start believing this. Where the hell did I come from?”

Jeannie looked thoughtful. “Something you said has been on my mind: ‘I don’t have any brothers or sisters.’ From what you’ve said about your parents, they seem like the kind of people who might want a house full of kids, three or four.”

“You’re right,” Steve said. “But Mom had trouble conceiving. She was thirty-three, and she had been married to Dad for ten years, when I came along. She wrote a book about it: What to Do When You Can’t Get Pregnant. It was her first bestseller. She bought a summer cabin in Virginia with the money.”

“Charlotte Pinker was thirty-nine when Dennis was born. I bet they had subfertility problems too. I wonder if that’s significant.”

“How could it be?”

“I don’t know. Did your mother have any kind of special treatment?”

“I never read the book. Shall I call her?”

“Would you?”

“It’s time I told them about this mystery, anyway.”

Jeannie pointed to a desk. “Use Lisa’s phone.”