“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.
“Mitochondrial DNA was used to determine that skeletons recently exhumed in Russia were those of Czar Nicholas and his family.”
“How?”
“Mitochondrial DNA is only passed on through maternal lines.”
“The whole shooting match comes from Mommy?”
“Sorry to break that to you, Ryan.”
“My gender knows grout.”
“The researchers compared DNA from the Russian bones to DNA obtained from living relatives, specifically Britain’s Prince Philip.”
“Queen Elizabeth’s hubby?”
“Prince Philip’s maternal grandmother was Czarina Alexandra’s sister, so Alexandra and her children, and Philip, inherited their mitochondrial DNA from the mother of both Alexandra and her sister.”
“Back to the cats.”
“Hair cells have no nuclei, so no genomic DNA. But mitochondrial DNA is present in hair shafts.”
“Gagne referred to epithelial cells.”
“Saliva, skin, buccal, vaginal. You might find saliva on cat hairs as a result of grooming—e-cells are also found in urine and feces. I appreciate Gagne’s pessimism about e-cells in this case.”
“Piss-poor chance of finding any.”
“According to Gagne, the mitochondrial sequences from the Specter cat were identical to those from the septic tank hair.”
“Meaning the Paraiso victim had contact with the Specter cat.”
