6 Things I’ll Bet You Didn’t Know Are In Your DNA
In biology, DNA is presented as a neat, orderly double helix comprised of nucleotides, which determine our genotype and - along with environmental factors - our phenotype. Unfortunately, the DNA replicating in our cells right now isn’t comprised of the perfect right-handed spirals that we picture as the “building blocks of life” - in the words of Cracked.com, “[it] is more like an old scrapbook that someone has torn up, pasted back together, filled with old newspaper clippings about murder and then taken into the bathroom with them.”
So let’s take a look at the creepiest of what scientists think 98% of our DNA - as in, not the approximately 2% that codes for useful proteins - is made up of.
- Ancient Viruses. I’ve blogged about this before, but it’s so cool I’ll mention it again. While a “normal” virus works by invading a host cell and using cellular machinery to reproduce, retroviruses actually mix their own genetic material into the cell they’re invading. Scientists believe that endogenous retroviruses picked up by our distant ancestors found their way into the sex organs, and the new virus-hybrid DNA was passed onto offspring - which ultimately evolved into us, racking up virus-laden DNA over thousands of years. As a result, scientists estimate we now have 100,000 of these microscopic gate-crashers cluttering up our DNA - making up a whopping 40% of our entire genome. (Edit: As jtotheizzoe pointed out, viral DNA itself only accounts for about 8-10% of the genome, although that’s probably underestimated since a lot of it is hopelessly degraded. The 40% number comes from retrotransposons, like LINE elements, which are not viruses - although they may be ancestors of retroviruses.) Even more eerily, new research suggests there could be a correlation between unexpectedly high levels of a particular endogenous retrovirus and schizophrenia.
- "Dead" genes. Our DNA is also full of evolutionary relics that have not yet been completely edited out - so called “junk DNA”, or “dead genes.” There’s just one problem with that name, however - the genes aren’t actually dead. A common form of muscular dystrophy, FSHD, is caused by a "dead" gene present in all humans. But it’s only “dead” because it’s missing one specific sequence that allows it to be successfully transcribed; all it takes is one tiny mutation, and the gene is fully expressed. If you thought that was just a fluke, think again: A gene thought to put people at risk for Crohn’s disease was resurrected after 25 million years, and by what? Another retrovirus, of course!
- Neanderthal DNA. How on earth is 1-4% of our modern genome the same as that of a Neanderthal? The obvious answer is, “Oh, it hasn’t been edited out by natural selection yet”…except for, awkwardly enough, that same 1-4% is only found in people with European and Asian descent, and not those descending from Sub-Saharan Africa. Scientists’ hypothesis? Some of our early ancestors got it on in the Middle East 600,000 years ago after leaving Africa. Neanderthals weren’t our only inter-species coital experience, either - in 2010, researchers discovered another species, the Denisovans, and we apparently got funky with them, too.
- Your family tree. And, unfortunately, not always in a good way. A study in Sweden revealed a strange pattern in a rural community that had gone through periods of both famine and abundance in the 19th century. The study found that the grandsons of men who’d had childhoods coinciding with abundant years had a life expectancy of 32 years less than the grandsons of those who had experienced famine, with the grandsons’ earlier deaths caused mainly by diabetes and heart disease.
Insect-spread parasites. The assassin bug of South America is well known for sucking the blood of sleeping victims while pooping on their faces at the same time. While this is gross, it doesn’t have any affect on our DNA - until we scratch the bite. That causes the bug faeces to enter our system, carrying the parasite T. cruzi - the cause of Chagas’ disease. Being a discerning and ever-questioning scientist, you’re probably thinking: Hey, wait, that’s not right! That’s not genetic! The scary part is it might be. Researchers who deliberately infected chicken eggs with T. cruzi and then tested the offspring of the infected chickens that emerged found that not only did those chickens have the parasite DNA, but so did their offspring, and so on.
Your Long-Lost Twin. In very rare cases, one of two twins in the womb will end up effectively killing the other in order to obtain more resources and nutrients for itself. In even rarer cases, the surviving twin can end up absorbing its dead twin’s DNA - a condition known formally as “chimerism.” In 2002, a woman named Lydia Fairchild submitted DNA tests for her three children as part of a welfare claim, only to have the results prove that genetically, she wasn’t the mother. Since DNA is considered the gold standard of medical evidence, she was accused of somehow stealing the children, even after the poor woman gave birth to another “nonrelated” child right in front of a social worker. Finally, more extensive testing unlocked the mystery: Her ovaries had a different set of DNA than her bloodstream. In other words, she’d given birth to her dead sister’s children. Hers wasn’t an isolated case, either: a woman getting typed for a kidney transplant found out that two of her sons belonged to a dead sibling, while a teenage boy being treated for an undescended testicle was found to possess an ovary from his dead sister.
Image: Computer simulation of DNA unwinding.