Just How Much is Enough?

 

While I am casting about for various ways to bring my fifth great-grandfather to life—at least for family history purposes—it occurred to me that while I might not find the documents I hope to locate on John Carter's life, there is one other source I could consult: his DNA. After all, within each one of us is a record of our ancestors, both known and unknown, which can provide more information.

But just how much is enough? After all, autosomal DNA is only so powerful; it loses its strength with every generation removed from us. Could my DNA actually include any genetic material from a relative as distant as a fifth great-grandfather? Besides, what is the likelihood that any other sixth cousin—for that is the degree of relationship we're likely to seek in these match cases—might share that same shred of genetic material with us?

I jumped over to my ThruLines readout at Ancestry.com to see if there were any entries for John Carter. Surprisingly, not only were there any—there were sixty five! Before we take a closer look at these assertions, though, let's step back and take in a few statistics. According to a study mentioned at the International Society of Genetic Genealogy's wiki, the probability that a fifth great-grandparent—like my John Carter in colonial Virginia—would share zero detectable DNA with me hovers at 4.95%. In other words, there's a good chance some of John Carter's genetic material is still speaking through me, even in such a distant relationship.

Taking this one step further—and flipping my optimism on its head—the probability of no detectable DNA relationship between, say, a sixth cousin and me would be 89.9%. In other words, while there is some trace of John Carter's DNA pattern in me, finding another cousin in my generation would be rather rare.

Let's look at this from another angle. ISOGG also provides statistics on this same question, based on reports of three well-known testing companies. Since I'm using my Ancestry.com test for comparisons right now, let's see what Ancestry reports: the probability that two sixth cousins would share enough DNA for a relationship to be detected sits at 11%. Not a solid number, but still a chance that there might be another Carter cousin among the millions who have tested their DNA at Ancestry.com.

But sixty five matches? All for John Carter's descendants? Right next to John Carter's name in my ThruLines readout was that of my fifth great-grandmother, Hannah Chew. I took a look to see how many matches and I shared her genetic signature. Usually, that number is approximately the same for a husband and wife, but in this case, I had fifty four matches linked to Hannah Chew, as opposed to John Carter's sixty five.

Naturally, there could be a good reason for that discrepancy: while I might share John Carter's genetic material with that many people, those matches who descend from John's first wife would be a half-relationship to me. Of course, that would be a relationship even more removed than those descendants of both John Carter and Hannah Chew—and less likely to show up in my match list. But it would still be possible that the larger number of matches showing for John than Hannah has been represented in a half-relationship.

Granted, some of those matches share a very tiny segment of genetic material with me. Some, once we check the paper trail, will turn out to be bona fide relatives. Others might be related, but through some other ancestor we haven't yet discovered. And some might share such a small amount of genetic material as to border on coincidence, not relationship.

We'll take a look at the particulars tomorrow to see if any of these matches lead to helpful discoveries.