Hey—Check Out Those Genes!
There’s an old saying, “If you don’t know where you’ve come from, you can’t know where you’re going.” We used to rely on paper birth certificates, marriage licenses and memory to help discover where we’ve come from; but paper gets damaged, people are fallible, and memories fade. Leave it to biotech to come up with a better way. New heritage-hunting techniques come courtesy of one our greatest scientific achievements: the sequencing of the human genome.
We now have tools to genetically examine ourselves and our past. The ability to decode genetics at modest expense allowed the DNA testing industry to develop. Companies such as 23andMe, Ancestry.com, and National Geographic sell genetic tests online. These kits, consisting of a plastic tube, a baggy, and a postage-paid mailer, promise biological insight into the past. But what about the science? This Biotech Primer Weekly overviews commercial DNA testing and how it works.
A Dash of Difference
Every human being alive shares an astounding 99.9% of their DNA! Only one tenth of one percent of genetic “stuff” accounts for a planetful of differences. That’s because our genome consists of an astonishing three billion-plus building blocks—the famous As, Cs, Ts and Gs.
Areas of variation in genes are called genetic markers. Geneticists and others use genetic markers to assess the probability that two people share a common ancestor. The more genetic markers two people share, the more likely they are related. The only people with the same exact genetic markers are identical twins.
Alphabet Soup: SNPs And STRs
The two major types of genetic markers are SNPs and STRs. In the alphabet soup of genetics, “SNP” is short hand for “single nucleotide polymorphism.” It simply means that one of the three billion building blocks that make up a person’s DNA has been switched. In other words, an A has been swapped with a G.
“STR” stands for “short tandem repeat.” These are short sequences of DNA that repeat from five to fifty times. The number of times a particular STR repeats varies from person to person.
This DNA Is Not That DNA
SNPs and STRs provide different types of information depending on their DNA origin. We humans have different types of DNA:
- The sex chromosomes are the X and the Y chromosomes. Men have one Y chromosome and one X chromosome; women have two Xs only. Thus, the Y chromosome contains DNA information about paternal ancestry.
- Mitochondrial DNA (mtDNA) is found in tiny compartments called mitochondria that convert sugar to energy in cells. Only woman pass on mtDNA because during fertilization the DNA in sperm mitochondria is quickly destroyed. Mitochondrial DNA can help trace maternal ancestry.
- Autosomal DNA is all the genetic material that is not found in the sex chromosomes and in mitochondrial DNA. Autosomal DNA is found in the pairs of autosomal chromosomes numbered one through 22. These 22 pairs come from both mom and dad – one copy from each. Autosomal DNA offers generic information about both the maternal and paternal lineages.
Not only are types of DNA inherited differently – some from mom, some from dad, some from both – but the rate at which their DNA sequence changes from generation to generation also varies. This is because mtDNA and Y chromosomal DNA change only through random mutation of their As, Cs, Ts and Gs. Change can take centuries before a noticeable difference appears. By comparing genetic markers from mtDNA and Y chromosomal DNA with those of indigenous peoples from various parts of the world, it’s possible to estimate where your ancestors hailed from way back when.
Making Family Connections
Each person shares half their genetic markers with siblings, parents, and any children they have. We share about a quarter with grandparents or grandchildren, aunts and uncles, nieces and nephews, and half-siblings. The amount of DNA we have in common with relatives diminishes by half in each successive generation. By seven generations back, the amount of DNA shared among relatives is less than one percent.
So, autosomal DNA is useful in trying to make connections between living relatives or determining how far back you share an ancestor with someone. However, it doesn’t shed light on “deep ancestry,” or the region of the world our ancestors came from centuries or even millennia ago.
Cocktail Fodder: Same Family, Different Ancestor?
Can siblings’ ancestral origins be different? The surprising answer is yes! Imagine that a small percentage, say 10 percent, of a woman’s DNA contained genetic markers indicating Mongolian ancestry. Because each egg a woman produces only carries half her DNA only some will carry Mongolian markers. Therefore, one sibling could show Mongolian ancestry, while another doesn’t.
Our Possibly Hairy Past
What’s one of the most intriguing secrets genetic testing can reveal? Just how much of a Neanderthal you really are! Our smaller, sturdier cousins’ genome was published in 2010, based on some very well-preserved DNA taken from bones found in a Croatian cave. The evidence suggests that early humans mated with Homo neanderthalensis. Thanks to DNA testing, we can find out just how cozy our distant relatives were with Neanderthals. The average person is about five percent Neanderthal.
Today’s technology makes it possible for the curious to shed light on their DNA. The databases of genetic markers are growing apace, making it easier to uncover long-lost relatives, for better or for worse. So how about it? Are you ready to find out who you really are, biologically speaking?
Emily Burke, PhD has worked in biopharma for 20 years, gaining science writing experience at The Scripps Research Institute and Ionis Pharmaceuticals. As a Ph.D. molecular biologist, she is passionate about advancing the public’s understanding of science. In addition to being a self-proclaimed “science geek,” she is regularly asked to speak at international scientific meetings. When not teaching and writing the WEEKLY for Biotech Primer, Dr. Burke swims with her swim club and performs regularly on the improv circuit in San Diego.