Why Siblings Can Have Different Ancestry DNA Results

We have the same parents... Shouldn't we have the same ancestry?

This is a frequent question that pops up among takers of Ancestry DNA tests...

My sibling and I have the same parents and we each inherited half their DNA... Shouldn't our ancestries be the same?

This is a question we've addressed in previous blog posts, but today we're going to devote one whole blog post to the answer!

Understanding DNA

Let's start with the basics...

DNA, or deoxyribonucleic acid if we're getting technical, is a spiraling, double-stranded molecule that holds the instructions for creating...you! It guides your growth, development, and provides a blueprint for how your body should function.

Half of your DNA comes from your mom and half from your dad, carried on structures called chromosomes. You inherit 23 chromosomes from your mom and 23 from your dad for a total of 46.

Picture chromosomes as the chapters in a book, a book of life, that is. 

Now, within each chapter, or chromosome, there are thousands of sentences, paragraphs, and sections. These are your genes, and they hold the instructions for everything about you - from the color of your eyes and hair to the shape of your nose and your height. It's your genes that make you uniquely you.

But here's where things get even more fascinating. When a baby is conceived, they don't receive a photocopy of their parents' chromosomes. Instead, there's a clever reshuffling process at work. The chromosomes from each parent break apart and swap sections, creating a new, unique set of chromosomes to pass on.

Think of it like creating a new recipe. You have ingredients from your mother and from your father. When a baby is conceived, it's not that you simply get half the ingredients from your mom and half from your dad. Instead, each parent's ingredients are diced, mixed, and stirred together to create a one-of-a-kind recipe - that's you!

Now, before we get too far ahead of ourselves, let's talk about DNA testing...

The Science Behind Ancestry DNA Tests

Here's an often-misunderstood fun fact: Most Ancestry DNA tests (including the ones at CRI Genetics) don’t read ALL of your DNA. That would be like trying to read the entire collection of the Library of Congress and most of it wouldn't be relevant to Ancestry!

Instead, Ancestry DNA tests usually focus on certain landmarks in your DNA called markers. Picture them as a sort of genetic breadcrumb trail that leads back through your ancestral past.

These markers act as signposts scattered throughout your genetic landscape. Some of these signposts can be traced back through countless generations, creating an ancestral map that connects you to specific regions and populations around the world. For instance, a certain marker might be common in individuals of Scandinavian descent, while another could be prevalent in those with West African heritage.

Here at CRI Genetics, we use something called autosomal testing for your Ancestry breakdowns. This is a method that investigates genetic markers scattered throughout your autosomes (the 22 pairs of non-sex chromosomes).

When you send in a sample to CRI Genetics, our lab technicians extract your DNA and scan it for these unique markers. We compare your markers to reference populations - groups of people from specific regions whose genetic markers are well-known. If you have a lot of signposts in common with a reference population, you likely have ancestors who came from that same region!

Now, let's explore how siblings can sometimes have completely different ancestry results!

Genetic Shuffling: A Game of Chance

In a process called meiosis, your parents' DNA is shuffled like a deck of cards before being passed onto you. This shuffling, or genetic recombination, creates a unique genetic blueprint.

It's called meiosis, and it's the process that ensures you get a mix of your parents' genes—but not the exact same mix your siblings do. Intrigued? Let’s dive into the details.

Imagine DNA as a library. Your mom has her library, and your dad has his. Each book in the library is a chromosome, and every sentence inside the book is a gene. When you were conceived, instead of receiving an exact replica of your parents' libraries, you received a unique collection of books, some from your mom's library, some from your dad's.

In more scientific terms, we humans have 46 chromosomes—23 from mom and 23 from dad. But here's the catch: the 23 you get from each parent aren't exactly the same as the 23 they got from their parents. This is due to our friend meiosis.

During meiosis, each parent’s 23 pairs of chromosomes line up and swap sections in a process called recombination. Picture this as a cosmic dance where partners (chromosomes) twirl, split, and rejoin, thereby exchanging parts. Once this dance is done, the chromosomes separate into sperm or egg cells, each carrying a unique set of 23 chromosomes.

It's the unpredictability of this genetic shuffling and dealing that results in each sibling being genetically unique, influencing how much of each ancestor's DNA they inherit, and thereby explaining why siblings can get different results from their ancestry DNA tests.

I Still Don't Get It -- Why Can Ancestry Results Differ Between Siblings?

Let's return to the card game analogy mentioned earlier...

Your parents' DNA is like a deck of shuffled cards. Each card in the deck is a gene from either your mom or dad.

When you were created, you and your sibling drew from this same deck of cards. You were dealt half the deck and your sibling was dealt the other half. 

Yet you ended up with different hands, as you would expect from ANY card game. 

In the case of DNA, meiosis ensured that the hand dealt to you was unique.

Even though you both have the same biological parents, you don't have exactly the same genes (except in cases of identical twins). Therefore, you won't have exactly the same genetic ancestry.

Let's imagine a scenario: you have a great-great-grandmother who hailed from the lush green landscapes of Ireland. Over the years, her genes have journeyed through generations, passing from parent to child, always reshuffling with each new chapter. Her Irish heritage is marked by certain genetic signposts, or markers, that can be identified by an ancestry DNA test.

Now, because of the randomness of genetic shuffling during meiosis, you might have inherited a substantial amount of these Irish genetic markers. Your sibling, however, may have inherited fewer of them due to a different shuffle of the genetic deck. As a result, when both of you take an ancestry test, your results may show a higher percentage of Irish heritage compared to your sibling’s.

This difference is not a fault of the ancestry test or an error in the reading of your genes. It's simply an illustration of the beautiful and complex game of chance that is genetics. Just as every book has different chapters, every sibling tells a different part of the family story through their DNA.

In essence, it's the combination of this genetic lottery and the interpretation of genetic markers by ancestry DNA tests that leads to siblings having different percentages of ancestral regions. You and your siblings are chapters in the same book, but you each tell a slightly different version of your family's story.

Remember, these tests don’t measure the depth of your cultural heritage or the richness of your personal connection to your ancestors. They merely give you a fascinating glimpse into the genetic journey that helped shape who you are today. As we continue to uncover the wonders of our genetic code, one thing is clear: our family stories are far from simple, and that’s what makes them so intriguing.