Linkage and Recombination
Relatedness

I loved reading about what each parent hands down genetically. I had always been taught that we inherited from all 4 grandparents, about 25% from each. But now I'm a little confused. How much DNA you inherit from each grandparent? Do boys get more from their paternal grandfather, while girls get more from their paternal grandmother?

— A middle school teacher from IN

January 30, 2020

You’re right! Genetic information is passed down from our grandparents to our parents, and then from our parents to us. So your family makes you who you are, genetically speaking.

You are a combination of genetic information of all four of your grandparents. But that does not mean that you necessarily have an equal contribution from all of them. However, on average, you do have about 25% of your genetic information coming from each grandparent.

Chromosomes and DNA

Your body is made of cells that contain your genetic information, or DNA. DNA gives instructions to your body to develop and function, and it makes you who you are. 

DNA is organized in our cells into long pieces called chromosomes.

Most of us have 23 pairs of chromosomes, for 46 in total. You get one of each pair from your mom, and the other one from your dad! That means you get exactly half your DNA from each parent.

22 of these chromosome pairs are the same between men and women. These are the autosomes. The last pair are the sex chromosomes, because they determine if you are a female (two X chromosomes) or male (an X and a Y chromosome). 
If we took a picture of one of our cells, this what our chromosomes would look like:

At the same time, your parents each have an even mix of DNA from both of their parents. 

So how much DNA do you get from your parents’ parents? Do you get some chromosomes from one grandparent, and other chromosomes from another, for ¼ from each? Is it all mixed up, so that you get exactly 25% of DNA from each?

Turns out it is a little more complicated than that. While it’s true you get ~25% of your DNA from each grandparent, the exact fraction that we receive from our grandparents is governed by chance. 

DNA gets shuffled each generation

I just mentioned that your parents received half of the genetic information from each of their parents. And then they pass this genetic information on to you.

But before this genetic information is passed down, it gets shuffled in an event called genetic recombination. Genetic recombination means that your chromosomes aren’t purely from one grandparent or the other one. Chromosomes get mixed and matched before they are passed on. 

This means that for a specific chromosome that you inherit from your mom, you are probably receiving a mix of DNA from both your maternal grandparents. And for that same chromosome you inherit from your dad, it’s likely a mix of DNA from both paternal grandparents. Therefore, you almost certainly have different genetic contributions from your four grandparents.

And this happens regardless of the genders! Boys and girls are equally likely to get DNA from their grandmother as they are to get DNA from their grandfather.

Take a look at a real example of the DNA that two siblings share with each of their grandparents. As you can see, they did not inherit exactly 25% of genetic information from each grandparent. Also, the amount of DNA that each sibling received from each grandparent slightly varies!

 
Click to view larger version of each image

Both siblings inherited chunks of DNA from each of their grandparents. But they didn’t always inherit the same chunks. Genetic recombination shuffled the DNA a bit differently for each of them!

That random shuffling explains why they share different amounts of DNA with each grandparent. 

Exception to the rules: sex chromosomes

But there is a pair of chromosomes that don’t get inherited evenly from all 4 grandparents! This is the last pair of chromosomes: the sex chromosomes. 

Only boys have Y chromosomes. This means that a male can only inherit the Y chromosome from his father, and his father could have only inherited from his father. Therefore, males always inherited their Y chromosome from their paternal grandfather. 

Take a look back at the grandson’s DNA. His Y chromosome is entirely from his paternal grandfather!

On the other hand, all males inherited their X chromosome from their mother. Notice that the grandson’s X chromosome is a combination of both maternal grandparents.

In contrast, the granddaughter has two X chromosomes. The one from mom is a combination of both maternal grandparents, just like in her brother. But the X chromosome from dad is entirely from the paternal grandmother. 

Since the father inherited a Y chromosome from the paternal grandfather, neither grandchild inherited any X chromosome DNA from the paternal grandfather.

Exception to the rules: Mitochondria

There is still one small part of our genetic information that we haven’t talked about. This is the one that we receive via the mitochondria. And mitochondrial inheritance does not obey the classic rules of genetics! 

Mitochondria are the powerhouses of the cell. They act as a digestive system for the cell, taking in nutrients and creating energy. 

Mitochondria have their own, separate DNA: the mitochondrial DNA. This DNA contains only 37 genes, but it’s pretty important. It helps keep your mitochondria working! 

Mitochondrial DNA is only inherited from your mom. The sperm’s mitochondria are lost during the fertilization process, leaving only the mitochondria from the egg. This means that everyone’s mitochondrial DNA comes from their mom, who got it from her mom, and so on. 

Notice that both grandchildren have mitochondrial DNA entirely from the maternal grandmother.

So how much do you get from each grandparent?

The percentage of DNA that you share with each grandparent is around 25%. 

It’s true there are some pieces of DNA that are not passed on evenly from all 4 grandparents. But they overall make up a very small percentage of your total DNA. These exceptions don’t affect the totals very much.

But you probably don’t have exactly 25% from each grandparent. This is mostly due to genetic recombination and random chance! 


By Marina Sumarroca, Stanford University


The images shown above are created from 23andMe data. 23andMe will show you how much DNA you share with a relative, and where that DNA is located. 

This family tested a brother and sister, and 3 of their grandparents! We synthesized all this data together to create the images for this article.

For more on how much DNA you might share with a relative, we recommend this tool: https://dnapainter.com/tools/sharedcmv4

Linkage and Recombination
Relatedness