Can Siblings Have Completely Different Genes?

Many biological siblings are taking DNA tests and finding themselves very confused once they compare results with one another.

It can be an eye-opening experience for families to take DNA tests because it gives a wider view of your family’s ancestry and genetic background.

However, why do some siblings have varying results if they’re closely related?

As siblings with the same mother and father, you each received DNA from both parents.

It is only logical to conclude that you and your siblings would have a lot in common with the same source of DNA. The truth is, it’s not that simple. Keep reading to learn more.

The Family Mismatch

It is true that we receive 50% of our DNA from our mother and the other 50% from our father. So, why do some siblings see varying degrees of ethnicities between them? For starters, it all began when the sperm met the egg. 

If your biology from highschool is foggy, here is a refresher. When someone’s body creates a sperm or an egg, a process begins called genetic recombination.

Essentially this means that the cells engage in some serious reshuffling. Genetic recombination cuts the normal number of chromosomes that a cell possesses in half, going from 43 to 23.

After this, the chromosomes form a complete genetic package when the sperm and egg combine during the fertilization process. 

Each time this genetic recombination occurs, the bits of genetic information transferred is different.

That is why you and your sibling both get 50% of your DNA from your mom and 50% from your dad. The parts of the DNA inherited vary with each sibling. 

Can One Sibling Have A Different Ethnicity?

Some siblings that take DNA tests have discovered that one of them has a higher percentage of one ethnicity than the other.

As we’ve learned, genetic recombination accounts for a lot of this. Depending on what you inherited from each parent, you may find your 15% more Italian than your brother.

This does not mean you are from different parents.

Some siblings are surprised to find they look wildly different. It is even possible that one sibling can have dramatically different skin, eye, and hair color than the others.

For example, if you take an interracial couple, it could happen that one child inherits blonde hair and blue eyes from his mom and another child has darker skin and brown eyes that he inherited from his dad. 

That’s because these features are defined by our DNA.

Which genes get turned “on” and “off” can have a dramatic effect on your appearance.

Which Parent Does The DNA Come From?

If your brother looks exactly like your dad, and you’re the spitting image of your mom, that does not mean that you received more DNA from your mom than your dad.

Both you and your brother inherited 50% of your DNA from your mother and 50% from your father. 

The way that DNA mixes and how the function of the specific gene, is how you get differences between siblings.

Two people can share the same genes, but the level to which that gene is “turned on” can be different between children. 

Think of it this way, you and your siblings’ DNA are each represented by a row of lamps.

The lamps are the same, but they vary in degrees of brightness when turned on. Due to these differences, each row will cast a different amount of light.

Your DNA functions quite similarly, which results in the discrepancies between siblings.

50% Shared DNA

Humans actually share around 99% of DNA with one another. While that sounds like we’re all basically the same, that 1% counts for a lot.

The change in one gene does not change the DNA recipe all too much. These differences are called single nucleotide polymorphisms, also known as SNPs.

If someone claims that 45% of their DNA is shared with their sibling, what they are actually saying is that they share 45% of all these SNPs.

What About Twins?

Twins are an interesting exception to what we have discussed so far. When identical twins are conceived, one zygote (which is formed by a sperm and an egg cell) splits in half, resulting in two different fetuses.

Because identical twins are the result of the same zygote, they will share 100% of their DNA.

In contrast, fraternal twins share around 50% of genetic variants, which is why they often look so different. 

Just because identical twins share the same DNA, does not mean that they are exactly alike.

Identical twins have different weights, personalities, features, and even food preferences, all of which are a result of genes and the environment they live in.