Once inside the lab, Pendergast took up a position against a table in the center of the room and chatted with the scientists, Buchholtz and Turow. D’Agosta admired how easily the Southerner could take charge of a scene.
“My colleague and I would like to understand the DNA sequencing process,” Pendergast was saying. “We need to know how you arrived at these results, and whether any further analysis might be called for. I’m sure you understand.”
“Certainly,” said Buchholtz. He was busy and small and as bald as Mount Monadnock. “My assistant, Dr. Turow here, did the analysis.”
Turow stepped forward nervously. “When we were given the sample,” he said, “we were asked to identify whether it had come from a large carnivorous mammal. Specifically, a big cat. What we do in such a case is compare the DNA in the sample to the DNA of, say, five or six species that are likely matches. But we would also select an animal that was definitely not of the sample, and we call this the outgroup. It’s a kind of control. Am I making sense?”
“So far,” said Pendergast. “But go easy on me. I’ m a child in these matters.”
“We usually use human DNA as the outgroup, since we’ve mapped so much of it. Anyway, we do a PCR—that is, a Polymerase Chain Reaction—on the sample. This causes thousands and thousands of copies of the genes to be made. It gives us a lot to work with, you see.
He pointed to a large machine with long clear strips of Plexiglas attached to its flanks. Behind the strips were dark vertical bands arrayed in complicated patterns. “This is a pulsed-field gel electrophoresis machine. We place the sample in here, and portions of the sample migrate out along these strips through the gel, according to their molecular weights. They show up as these dark bands. By the pattern of bands, and with the aid of our computer, we can figure out what genes are present.”
He took a deep breath. “Anyway, we got a negative reading on the big cat genes. A very negative reading. It wasn’t anywhere close. And to our surprise, we got a positive reading on the outgroup, that is, Homo sapiens. And, as you know, we identified DNA strands from several species of gecko—or so it appears.” He looked a little sheepish. “But even so, most of the genes in the sample were unidentified.”
“So that’s why you presumed it was contaminated.”
“Yes. Contaminated or degraded. A lot of repeated base pairs in the sample suggested a high level of genetic damage.”
“Genetic damage?” asked Pendergast.
“When DNA is damaged or defective, it often uncontrollably replicates long repeating sequences of the same base pair. Viruses can damage DNA. So can radiation, certain chemicals, even cancer.”
Pendergast had begun to roam around the laboratory, examining his surroundings with an almost catlike curiosity. “These gecko genes interest me a great deal. Just what do they mean exactly?”
“That’s the big mystery,” said Turow. “These are rare genes. Some genes are very common, like the Cytochrome B gene, which can be found in everything from a periwinkle to man. But these gecko genes—well, we don’t know anything about them.”
“What you’re really saying is the DNA didn’t come from an animal, right?” D’Agosta asked.
“Not from any large carnivorous mammal we know of,” Buchholtz answered. “We tested all the relevant taxa. There are not nearly enough matches to say it came from a gecko. So, by a process of elimination, I would say it probably came from a human. But it was degraded or contaminated. The results are ambiguous.”
“The sample,” said D’Agosta, “was found in the body of a murdered boy.”
“Ah!” said Turow. “That could easily explain how it was contaminated with human genetic material. Really, it would be much easier for us if we knew things like this beforehand.”
Pendergast frowned. “The sample was removed from the root canal of a claw by the forensic pathologist, as I understand it, and every effort was made to prevent contamination.”
“All it can take is one cell,” said Turow. “A claw, you say?” He thought a moment. “Let me advance an idea. The claw might be from a lizard that was heavily contaminated by blood from its human victim. Any lizard—not necessarily a gecko.” He looked at Buchholtz. “The only reason we identified some of the DNA as gecko is because a fellow in Baton Rouge did some research a few years ago on gecko genetics, and logged his results in GenLab. Otherwise it would have turned up unknown, like most of that sample.”
Pendergast looked at Turow. “I’d like further tests done to tell us just what those gecko genes do, if you don’t mind.”
Turow frowned. “Mr. Pendergast, the chance of a successful analysis is not all that high, and it could take weeks. It seems to me the mystery’s already been solved—”