In our bodies we contain two different types of DNA. There is the bog-standard (or, nuclear, because it’s in the nucleus) DNA which is a combination of half of your mother’s and half of your father’s DNA which you inherited when one of your father’s sperm fused with one of your mother’s eggs. And then there is Mitochondrial DNA which is different to nuclear DNA in that we inherit it directly from our mother alone as a part of the egg which is swarming with hundreds of thousands of mitochondria . Nuclear DNA fulfils the role of building our bodies and mitochondrial DNA provides the means for mitochondria to generate the energy a cell needs to do the things it does (i.e. dividing or making stuff). When cells divide they make a duplicate of your DNA and, inevitably, the mitochondria end up dividing themselves up between the newly-formed cells where they will recombine with each other to keep the numbers up.
If you are a mushroom, a plant, or an animal (from jellyfish to wetas to elephants) you will have mitochondrial DNA and if your species uses sex for reproduction you will likely have inherited it from your mother.
Every time nuclear DNA is recombined during procreation and every time mitochondrial DNA is recombined within the cells of your body little mutations can occur. Most of these mutations are neutral in that they don’t inhibit or enhance the functioning of the DNA.
What this means is that, while our nuclear DNA will pick up on average 128 mutations (out of ~3,000,000,000 base pairs) during sexual reproduction, mitochondrial DNA have more mutations (out of only ~17,000 base pairs in mammals) without the added complication of sexual recombination and this makes it much easier to compare mitochondrial DNA between people and, therefore, make reasonably accurate predictions for just how closely maternally related they are by comparison of their mitochondrial DNA alone.
Let’s say we take your mitochondrial DNA and compare it with your sibling’s. We will find that the two samples are almost, but not quite, identical. This is because there have been a handful of mutations in the time that you’ve had your mitochondrial DNA to yourselves. Now if we take your sample and compare it to a maternal cousin (i.e. you mother’s sister’s children) you will find that, while still almost identical, a few more changes will have crept in. You will notice a pattern of increasing change occurring as you compare the mutations of maternal second cousins, third cousins, fourth cousins and so on and so on.
Now, once you have established what rate of change you can expect between maternal relatives you can go ahead and test your neighbour. If you find that your neighbour’s difference in mutations are only about those of, say, your maternal second cousin then you will be able to fairly confidently predict that you have just met a previously unknown second cousin. If, however, you compare you neighbour’s mutations and they are larger than what you would expect from a close relative you should be able to make a prediction for how many generations ago it was that you shared a common grandmother based on the rate of change.
You may have heard before of a ‘mitochondrial Eve’ that was talked about a few years back. They got to this conclusion by testing as many diverse people in the world as they could think of and compared their mitochondrial DNA to see what was the greatest difference they could find. It turns out that the most remote common maternal ancestor they could identify lived around 8000 generations and perhaps 170,000 years ago. The exact numbers are, understandably, still hotly disputed but the fact of the genetic relationship is sound.
Some people have taken this to mean that this ‘Eve’ was actually the first human but they’ve missed the point. We could have done a ‘mitochondrial Eve’ analysis for just the people of a particular village in South America and found that their mitochondrial Eve was only 50 generations ago. Conversely, we could have done a mitochondrial Eve analysis of humans and chimpanzees and discovered that they are around 250,000 generations and 7 million years ago (as, indeed, they have found).
There are many other ways to measure the relatedness between animals including the Y chromosome (which is passed exclusively from father to son) and a plethoria of nuclear DNA comparisons. But I’ll leave those explanations to people who actually know what they are talking about. If this kind of stuff interests you like it does me, I wholeheartedly recommend The Ancestor’s Tale by Richard Dawkins.