Grave Secrets

“Which samples match?” Ryan asked.

When the receptionist answered, I asked for Gagné.

“That’s what I’m anxious to know. The Eduardo cat’s been ruled out.”

“Why?”

“Persian.”

“Poor Fluffy.”

“Buttercup.”

Gagné came on the line.

“Sorry,” I said. “You caught me underground.”

“You sound like you’re still down there.”

“I have you on speakerphone. Detective Ryan is with me.”

“Ryan’s on this?”

“All over it. Please repeat what you were saying.”

“I was saying that I went with mitochondrial DNA. Three of the samples looked O.K., but the hairs in the packet marked ‘Paraíso’ had no root or sheath with an appropriate follicular tag to enable genomic DNA processing. You told me to test everything.”

I had. But I’d meant Gagné could use the entire Paraíso sample, since the Guatemala forensics lab had retained hair for future testing. I had no idea Minos’s package contained other samples.

“I could have looked for epithelial cells on the Paraíso shafts, but given the context I doubted I’d find much,” Gagné went on.

“Cats have polymorphic regions in their mitochondrial DNA?” I asked.

“Just like humans. A feline geneticist at a cancer institute in the U.S. researches this stuff, has excellent stats on population variability.”

Ryan was holding a finger to his head, mimicked pulling a trigger. Linus Pauling he’s not.

“What was the match, M. Gagné?”

A paper rustled. I held my breath.

“The sample marked ‘Paraíso’ profiled like the sample marked

‘Specter.’”

Ryan stopped blowing smoke from his fingertip and stared at the phone.

“Meaning they were consistent?”

“Meaning they were identical.”

“Thank you.”

I disconnected.

“You can holster your weapon.”

Ryan dropped his gun pantomime and placed hands on hips.

“How can he be so sure it’s a match?”

“It’s his business to be sure.”

“The hair’s been in a friggin’ septic tank.” Ryan’s tone oozed skepticism.

“Do you know anything about DNA?”

“What I don’t know I have a feeling I’m about to learn.” He raised a hand, palm out. “The five-minute version. Please.”

“Do you know what a DNA molecule looks like?” I asked.

“A spiral staircase.”

“Very good. Sugars and phosphates form the handrails, and bases form the steps. How can I bring this down to your level?”

Ryan opened his mouth to object, but I cut him off.

“Think of the bases as Legos that only come in four colors. If there’s a red Lego on one half of a step, there’s always a blue Lego on the other. Green pairs with yellow.”

“And not everyone has the same color pattern at a particular place.”

“You’re not as dumb as you look, Ryan. When multiple variations exist for a sequence of steps, it’s called a polymorphism. When a position has extreme numbers of variants, maybe hundreds, it’s called a hypervariable region.”

“Like Manhattan.”

“Did you want this in five minutes?”

Ryan held up both palms.

“Variations, or polymorphisms, can occur in the sequence of colors, or in the number of times those colors are repeated between any two specific steps. You with me?”

“A particular fragment can vary in pattern or length.”

“The first technique that was adapted for forensic DNA analysis was called RFLP, Restriction Fragment Length Polymorphism. RFLP analysis determines variation in the length of a defined DNA fragment.”

“Produces that thing that looks like a grocery store bar code.”

“It’s called an autoradiograph. Unfortunately, RFLP requires better-quality DNA than many crime scene samples provide. That’s why PCR was such a breakthrough.”

“Amplification.”

“Exactly. Without going into details—”

“But I love it when you talk dirty.” Ryan reached out and touched my nose. I brushed his hand away.

“Polymerase Chain Reaction is a technique for increasing the amount of DNA available for analysis. A defined sequence of Lego steps is copied millions of times.”

“Genetic Xeroxing.”

“Except that with each round the number of copies doubles, so the increase in DNA is geometric. The drawback to PCR analysis is that fewer variable regions have been identified, and each tends to show less variation.”

“So you’re able to use PCR with crummier DNA, but the power of discrimination is lower.”

“Historically that’s been the case.”

“What’s this mitochondrial stuff?”

“RFLP and PCR—and there are other procedures—use genomic DNA, which resides in the cell nucleus. Additional bits of genetic material are found in the mitochondria, small compartments in the cell where respiration takes place. The mitochondrial genome is smaller, slightly over sixteen thousand bases, and forms a circle, not a staircase. There are two regions on that circle that are highly variable.”

“What’s the advantage?”

“Mitochondrial DNA is present in hundreds to thousands of copies per cell, so it can be extracted from small or degraded samples where the genomic DNA is long gone. Researchers have found mitochondrial DNA in Egyptian mummies.”

“I doubt your septic tank was built by pharaohs.”

“I was trying to make this understandable.”

I thought of a better example.