“Siblings can differ in their genetic ancestry results, here’s why you shouldn’t be concerned”
A common concern when families participate in genetic ancestry tests is why siblings can have such different ethnicity results. For example, one sister might have 30% Irish ancestry, but another is only 10%.
The reason behind this is that while siblings always inherit half of their DNA from each parent, they always inherit a different half , the result of a process called “genetic recombination”
A common concern when families participate in genetic ancestry tests is why siblings can have such different ethnicity results. For example, one sister might have 30% Irish ancestry, but another is only 10%.
We’re familiar with the fact that we inherit half of our DNA from our mom and half from our dad. Looking across our siblings, however, we can easily see that we don’t all look alike. Similarly, two siblings from the same parents can often have very different genetic ancestry results and vary in the exact proportions of certain ethnicities. The reason behind this is that while siblings always inherit half of their DNA from each parent, they always inherit a different half (except for identical twins). More technically, during egg and sperm cell formation, each parent’s DNA gets “shuffled-up” randomly in a process called genetic recombination, before being passed on to their child.
We can visualize the process of genetic recombination by imagining our DNA as a deck of playing cards. A person’s genome, all of their genetic information, is made of up of two decks—one deck of blue cards inherited from mom and one deck of red cards inherited from dad. Every time an individual creates a new egg or sperm cell, these two decks are shuffled together. At the end of this process, each egg or sperm cell carries a new randomly selected deck of 52 cards that will be passed on to the next generation.
Following these rules, a child inherits one recombined deck from each parent. Likewise, a parent always passes on a full deck of 52 cards, but how many blue cards are in the deck versus how many red cards, varies. At the genetic level, physical pieces of DNA are literally swapped within the genome, just like the cards in our analogy. This means that no egg or sperm cell in a parent is genetically identical and that siblings inherit a different component of their parents’ DNA from each other.
How does this impact you and your sibling’s ancestry? Imagine that your mom was 100% Irish, and your dad was 100% French. In our analogy, all your mom’s cards are Irish, and all your dad’s cards are French, so not matter how their own decks get shuffled, you and your siblings will always be 50% Irish. Your mom will always pass on a full deck of Irish cards. However, if your mom is, instead, 50% Irish and 50% English, when her DNA gets shuffled during recombination, the deck of cards she passes to one child may have more Irish cards in it than the deck she passes on to another child. Each child still gets half of their DNA, or a single full deck, from mom, but how much Irish ancestry is present in that DNA varies.
Genetic recombination is a random process and, because of this, the differences in ancestry between siblings can initially look extreme. However, this is actually very normal. Importantly, it is not the case that one sibling’s ancestry is more “correct” than another’s. All siblings carry important genetic information about their family’s history, since each sibling carries a different, but complementary, component. Combining information across siblings is the best way to fully reveal your family’s story.