Family history is much more than the experiences of a single generation—it includes an ancient story that is recorded in our DNA. Genetic ancestry tests can help people explore that story by offering information on ancestral migration routes and whether a person’s DNA contains remnants of extinct hominids.
It would be easy to assume that one person’s genetic ancestry results can tell a family’s whole story. But that’s not exactly true. In fact, two siblings who have the same parents might have different ancestry genetic test results! How is that possible? A combination of statistics, random chance, and genetics.
Determining a person’s ancestry from their DNA can be done with 3 different categories of DNA testing (all of which are used by ancestry products offered in the Helix Store):
- Y chromosome: The Y chromosome is a sex chromosome, meaning it helps determine the biological sex of a person—specifically, it causes a person to be biologically male. Y chromosomes are passed down from generation to generation through the male line of inheritance. This means the Y chromosome can be used to trace a person’s paternal ancestry back hundreds of thousands of years; however, biological females do not have Y chromosomes. Because of this, people without a Y chromosome won’t have access to its ancestry clues unless one of their biologically male relatives gets ancestry testing.
- Mitochondrial DNA: This type of DNA resides within the mitochondria in our cells and is passed to both male and female offspring, but only mothers can pass it on to the next generation. This DNA analysis can be used to trace a person’s maternal ancestry, potentially as far back as 200,000 years!
- Autosomal DNA: This type of DNA represents the majority of a person’s genome, encompassing everything that is not on a sex chromosome or located within the mitochondria. Mothers and fathers both pass on autosomal DNA to the next generation, which means that ancestry tests using autosomal DNA can give information about both maternal and paternal lineages. This type of DNA can be used to trace a person’s ancestry back several generations.
Ancestry tests can give people a representation of their genetic history. But the DNA an individual inherits represents only part of their family’s lineage.
So how can two siblings have different genetic ancestry results? To understand this, it’s important to recognize that most people have two copies of every chromosome—one copy from mom, and one copy from dad (except on the sex determining chromosomes for males, who have an X and a Y). When a sperm or egg is produced, the body will randomly select one chromosome from each pair to be passed on; some coming from the mom, and some from the dad. This process is depicted in Figure 1, where 6 chromosomes, or 3 chromosome pairs, are shown as an example. (Humans have 46 chromosomes in total, representing 23 pairs.) The randomness of this selection process means that even if someone had a thousand children and none of them were identical twins, all one thousand would be unique.
Let’s consider an example where two siblings (named Sami and Cady) order ancestry testing. Now let’s say that their dad’s grandmother has an Irish ancestry, while their grandfather had an African ancestry. If by some extreme chance Sami inherited all 23 chromosomes from her grandmother and Cady inherited all 23 from her grandfather, their ancestry would appear to be different. Sami’s ancestry results would indicate that she has an Irish background, while Cady’s would be African.
Figure 2. In this diagram, solid colors represent chromosomes from one of the respective grandparents. Circles indicate females, and squares indicate males.
Figure 2 is an extreme example of this which demonstrates how, through random chance, siblings can inherit chromosomes from different ancestors. In this example, if only sibling 2 is sequenced, and there is no historical documentation available, then the group of siblings wouldn’t know about their blue or pink ancestry. If all of them were to be sequenced, they would have a more complete understanding of their family’s genetic history. In reality, each sibling is statistically more likely to inherit some chromosomes from each ancestor (similar to how sibling number 4 is represented). The ratio of which chromosomes were inherited from which ancestral line is what determines a person’s ancestry.
What does it all mean? We’re incredibly unique, and even knowledge of the family tree may not tell the full story of our personal genetic ancestry. Modern ancestry tests can reach further back into our genetic history than ever before. As with written records, DNA test results from multiple people within a family can help to build a more complete picture.