CECE MOORE (Genetic Genealogist): We inherit our DNA from both of our parents: 50 percent from mom, 50 percent from dad. And they inherited it from their parents; and their parents, of course, inherited it from their parents.NARRATOR: Our parents each contribute about 50 percent to our DNA. And the same is true for them and their parents. So, the amount of DNA we inherit from any ancestor drops by half with each preceding generation.We also share DNA with anyone who shares a common ancestor with us: siblings, half-siblings, first cousins, second cousins and so on. The way that the D.T.C.s determine those relationships is by comparing people’s DNA. The amount that is shared is measured in a unit called centimorgans.CECE MOORE: The more centimorgans two people share, the closer they are related. And the fewer centimorgans they share, the more distantly related they are.NARRATOR: But with the D.T.C.s, a relationship to someone can’t always be determined just by counting centimorgans, because the numbers fall within ranges. You might share the same number with a cousin and a great-uncle, for example.ELLEN GREYTAK (Parabon NanoLabs): Just because you have an amount of shared DNA doesn’t mean you actually know, for sure, what that person’s relationship is. It’s just a probability, a spectrum of possible relationships.

Jenny searched for answers online and learned about the centimorgan - a unit of genetic linkage.