When DNA Isn’t Enough: Methylation, Forensics, and Twins – Part 1

By April 18, 2016Blood and DNA, ID

DNA evidence is often considered a “home run” in forensics. If you find readable DNA at the crime scene, and it matches a suspect, a correct conviction is almost assured. A DNA sample can often point to a single individual with ridiculous specificity — often 1 in a quadrillion or greater. But, what happens when someone else shares your DNA? This happens to be the case with identical twins, who share the same DNA profile. However, a team of scientists in Germany may have found a way to tell identical twins apart by their DNA.
 

Splitting Souls, a Myriad of Mutations

During early embryonic development, only a few cells are present. These cells quickly begin to differentiate into the different tissue types that they will become. As these cells divide to produce more cells, mutations can occur in the DNA. If a mutation occurs early on, it will be present in a larger number of the subsequent cells, and will be more easily detectable during the twin’s lifetime. This also means that the earlier the twins split, the less mutations they will have in common (thus, the more differences you can detect in their DNA).
 

The Role of Single Mutations

It has been suggested recently that a handful of single nucleotide mutations, or “SNPs,” can be found between twins. However, these SNPs aren’t so easy to find in a sea of 3 billion other nucleotides that make up a human genome. To find these few differences and find them reliably, the entire genome of both twins must be read several times over. In the case of the German scientists, their experiment results in 94-fold coverage, meaning they covered each of the 3 billion nucleotides 94 times. This must be done to ensure accuracy since at 3 billion nucleotides, even 99.9% accuracy will still result in 30 million errors.
 

The Missing Pieces?

At any rate, the scientists tested their new method on a set of twins, and it worked. In the end, twelve SNPs were identified between the twin brothers. Typically, one experiment is not considered to hold much weight in science, but this particular experiment is backed by strongly reinforced genetic theory, and the results were exactly what we would expect.
 

Not so fast…

So, case solved, right? Well, maybe not. It turns out that this method comes with a hefty price tag – over $100,000. This is far too costly to be practical in forensic case work. Of course, this price will go down as DNA sequencing prices continue to plummet in light of newer, better technology. Still, it will be many years before anything like this will be affordable. Furthermore, the instruments used in this method (net generation sequencing), though common in research science, have not been approved for use in court. That in and of itself can be a challenging obstacle to overcome, regardless of costs.
 
Your Turn: Perhaps in a few decades these issues will be resolved. Perhaps not. Either way, it might be a good idea to have a plan in the meantime. Have any ideas? We’d love to hear your comments. Leave your thoughts below.

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