What are the odds of inheriting no DNA from a great, great, great grandparent?
-A curious adult from Canada
December 7, 2011
This is a great question and probably one that a lot of people would be curious about. The quick answer is that the odds are pretty close to 100% that you have DNA from your great, great, great grandparent.
If you know anything about how our DNA is passed on, this might seem weird at first. DNA is passed down to the next generation in big chunks called chromosomes.
Every generation, each parent passes half their chromosomes to their child. If nothing happened to the chromosomes between generations, then there would be around a 1 in 8 chance that you would get no DNA from a great, great, great grandparent.
What most people forget, though, is that our chromosomes get mixed and matched before they are passed on. It is because of this "recombination" that your great, great, great grandparent's DNA is almost certainly still lurking in yours.
Mixing DNA Keeps You Connected
Most people have 23 pairs of chromosomes for a total of 46. One of each pair comes from mom and the other from dad. This is why we are 50% related to our moms and 50% to our dads.
Of course our parents got their chromosomes from their parents. This is why you are about 25% related to each grandparent.
If this is all there were to passing on our DNA, then this is what a typical couple of generations might look like:
At first this picture might seem a bit overwhelming but it isn't really. Each grandparent's chromosomes are a different color.
Mom ended up with half of one parent's DNA and half from the other (half green and half light blue). Same thing with dad except he ended up with half red and half dark blue in our picture.
When mom and dad have you, you get a mix of their chromosomes which are a mix of their parent's chromosomes. So you have green, red, light and dark blue chromosomes.
If this is how things worked, then there could even be a small chance you wouldn't be related to your grandparents. All's that would have to happen is something like this:
As you can see, by chance you happened to not get any light green chromosomes. You are totally unrelated to your maternal grandpa!
The odds of something like this happening would be very small...something like 1 in 8.4 million.* It is about the same odds as flipping a coin 23 times and getting all heads.
In fact, the two problems are similar. For a coin, on each flip you can get a head or a tail. For chromosomes, each generation you can get one from grandma or one from grandpa.
If we were to keep playing this game, what would the odds be that you'd get no DNA from one of your great, great, great grandparents? With a few assumptions, it would be about 1 in 8.*
This is not reality though. What really happens is that DNA is swapped between the two chromosomes in each pair before it is passed down. A single generation actually looks like this:
You didn't get any whole chromosomes from either parent. Each chromosome from mom has some DNA from her mom and some from her dad. Same thing with the chromosomes your dad gave you.
This means you don't end up with a 50% chance for a blue and a 50% chance for a red chromosome from dad. Or a 50% chance for a light blue and a 50% chance for a green chromosome from mom. Instead you get a chromosome that is about half red and half blue from dad and one that is half green and half light blue from mom. As you can see this happens for each of your 23 chromosomes.
This is actually how you are 25% related to your grandparents. Because of recombination, each chromosome you got from your mom is a mix of her parents' chromosomes and each chromosome you got from your dad is a mix of his parents' chromosomes. This means you will almost certainly get DNA from all of your grandparents.
And this holds true for later generations too. Here is how much DNA on average you'll share with each generation:
Because of recombination, you can't easily lose big chunks of distant relative's DNA. Instead it is slowly diluted away.
So on average you'll have about 3% of your great, great, great grandparent's DNA. There is a good chance it is a little more or less than this because the DNA swapping in recombination is pretty random. This means that sometimes you'll get a lot of one of the DNA from one chromosome in a pair and a little from the other.
But the odds of getting no DNA from a distant relative over 23 different chromosomes are very small. So small that you almost certainly have at least a bit of their DNA.
In fact, you have DNA from very distant ancestors. For example, scientists can still see Neanderthal DNA in people living today and Neanderthals died out 30 thousand years ago!
From all of this you might think that none of your DNA is unique...it all came from your ancestors. This isn't true. You have some of your very own DNA that none of your ancestors had.
Where'd this DNA come from? Not aliens or anything like that. No, they mostly came from mutations.
A mutation is a change in our DNA. It can happen because of environmental reasons like radiation, viruses or chemicals or because of a mistake when our DNA gets copied. Since these mutations change the DNA, it will change what we inherit from our parents. As a result we could have a bit of DNA that is different from our ancestors.
Of course this is in addition to the brand new mix of DNA that recombination gave us. You are unique and so is every one of your chromosomes.
* The way you figure out these odds is you apply this formula:
In this formula, x is the number of chromosome pairs that have to go a certain way (or the number of coin flips). Basically you are figuring out the odds that for each pair, a chromosome always comes from one grandparent.
In this case, it has to happen for 23 different chromosomes. When we plug 23 for x, we end up with the odds being 1 in about 8.4 million. And here are the rest of the numbers through the generations:
- Chances of no DNA from one grandparent: 1 in 8.4 million
- Chances of no DNA from one great grandparent: 1 in 4096 (assumes 12 passed down in last generation)
- Chances of no DNA from one great, great grandparent: 1 in 64 (assumes 6 passed down in last generation)
- Chances of no DNA from one great, great, great grandparent: 1 in 8 (assumes 3 passed down in the last generation)