15.8 Genealogical vs Genetic Family Trees
Genealogical family trees are not the same as genetic family trees. There can be many reasons for the differences. Perhaps there has been adoption or a misattributed parent event. Both may be known or unknown to family members who create genealogical family trees, and these are not the only circumstances that cause these types of conflicting data. People create trees based on their understanding and knowledge as they see the truth. Sometimes people intentionally create trees to hide the truth. However, mistakes or lies are not the only reasons genealogical and genetic trees may differ. Science also plays a part. Two siblings with the same parents will have differences in DNA. They come from the same family and their genealogical trees may match but genetically, one may inherit DNA from an ancestor that the other sibling does not share a genetic commonality with. Bettinger described it well when he explained the difference between them:
“It’s a common misconception that close relatives should share all the same genetic matches. Close relatives don’t always share the same DNA, and thus won’t always share genetic matches.… The likelihood of inheriting a segment of DNA from an ancestor decreases with each generation separating the test-taker and the ancestor. For example, a parent doesn’t pass down all his or her DNA to the next generation. Instead, you inherit just 50 percent of your father’s DNA and 50 percent of your mother’s DNA. Half of each parent’s DNA isn’t passed down to you. Therefore, you receive just 25 percent of each grandparent’s DNA on average… As a result, everyone has two family trees: The first is your genealogical tree, which is every ancestor in history that had a child who had a child who had a child who ultimately led to you… The second is the genetic family tree, which is made up only of those ancestors who gave you DNA. Because you only inherited DNA from some of your genealogical ancestors after about the seventh generation, only some of your genealogical ancestors are in your genetic family tree… Although siblings have the same genealogical tree, they’ll have different genetic trees (unless they’re identical twins). Similarly, you have a different genetic tree from each parent, although your genetic family tree is a subset of your parent’s genetic family tree.”1