Your Genealogical Family Tree Versus Your Genetic Family Tree: Inheritance Patterns and Why They Matter

As you delve deeper into your family history research, you'll soon discover there are endless ways to arrange your family tree. Learn more about the difference between a genealogical and a genetic family tree from one of our genetic genealogists.

“You have your mother’s eyes!” 

“You look just like your grandfather.”  

“You have your great-grandmother’s singing voice!” 

Many of us grew up hearing sayings such as these, about how our physical traits, talents, or personality quirks remind family members of generations gone before us. For example, I’ve often been told that I must have inherited my love of music from my maternal great-grandmother, who was a talented pianist. However, I certainly did not inherit my paternal grandmother’s talent as a painter and artist – those skills must have gone to my cousins. 

Using DNA to Create a Family Tree

As a genetic genealogist, I often think of the segments of DNA we inherit from our ancestors in the same way personality traits or areas of talent shine through across generations. Multiple descendants of a common ancestor all inherit slightly different portions of that shared ancestor’s DNA, just a person and their siblings may reflect different parts of their parent’s appearance as they age. There are traits we inherit and traits we do not. 

Genealogical DNA tests are one of the many resources family historians rely on to solve complex problems. Every person inherits basically half of their DNA from their parents, about one quarter from their grandparents, one eighth from their great-grandparents, and so forth. However, this division becomes less even with every subsequent generation. After five or six generations, there may be ancestors from whom a person inherits no DNA at all.  

In the following hypothetical genetic tree of a DNA tester, the first generation (“Joe”) shares 100 percent of their DNA with himself. Joe shares 50 percent of his DNA with his parent, but rather than an expected 25 percent shared DNA with each grandparent, Joe inherited a 21/29 percent split. At the great-grandparent level, each contributed 10 and 11 percent DNA, respectively. Notice how the sum of the DNA from the great-grandparents equals the total of their child, the tester’s grandparent.  

This tester therefore could not share DNA with any descendants of his depicted great-great-great-grandfather’s ancestors. If the tester has genetic cousin matches to descendants of his great-great-great-grandparents, all shared DNA would be from the great-great-great-grandmother. If Joe’s great-great-great-grandfather was a stubborn brick wall ancestor, no amount of searching Joe’s DNA matches would yield relevant cousins. 

However, let’s say Joe has a sister, Susan. Even though Joe and Susan are full siblings, they inherited different portions of their shared ancestors’ DNA. Susan’s genetic tree might look like this: 

Susan inherited six percent of her DNA from her and Joe’s mysterious great-great-great-grandfather, whereas Joe inherited no DNA from him! Susan could very well have key genetic matches on that ancestral line that could break down the family’s stubborn genealogical brick wall. However, if Joe had relied on his DNA results alone and not enlisted his sister to help with the research by taking a DNA test, he may never have found out why that ancestor seemed so elusive in his DNA. 

This principle can be extrapolated across varying degrees of cousins. A person and their first cousins share a set of grandparents, but each cousin inherited different portions of their grandparents’ DNA, second cousins share great-grandparents, and each second cousins inherited different portions of their great-grandparents’ DNA, and so forth. This is why it is essential to analyze the DNA results of multiple descendants of a research subject where possible. You never know which cousin’s DNA holds the key to unlocking a longstanding family mystery! 

If you'd like help analyzing and organizing your DNA test results to create your own accurate family tree, you can purchase a research project to work with our professional genetic genealogy team.