Frequently Asked DNA Questions

Q. How can I use DNA relatives to find a missing ancestor?

A. First, try to get parents, grandparents, etc. to get their DNA tested — the farther back the better. Note that it isn’t very useful to get a person’s DNA tested if both of their parents do so, too. Next, keep track of your segments of DNA (not just chromosome number) and which of your DNA relatives match with you on a particular segment. Be careful not to assume that two people match with you on the same segment just because the start and stop positions overlap. You have two homologues of each chromosome, so one person could be on your father’s side and the other person from your mother’s side. Instead, make sure that those two people also match with each other. For this reason it’s very good to get at least one of your parents to test their DNA. Once you’ve found more than one person with a family tree on a particular segment, look for a common ancestor (or more likely, ancestor pair) between the two. You can use your own tree for this, as well, if you think it’s accurate and well-filled. You can start to identify certain segments as likely from particular parts of your tree. That will allow you to focus on certain segments of interest. Gedmatch.com is probably the best site for comparing DNA relatives, however many people don’t have family trees there. Myheritage.com and 23andme.com have recently made this analysis much easier, although 23andme doesn’t have a system for storing family trees and you’ll find that many of your DNA relatives are not sharing the information you need to see on what segment you match with them. MyHeritage allows free uploads of kits from other sites and so does gedmatch. FamilyTreeDNA also allows free uploads, but has very limited functionality for people who do so. Crazily, the DNA site with the most users, Ancestry.com, has no chromosome browser — the only way, genetically, to see if you and multiple other people have the same common ancestor.

Q. Are centiMorgans or SNPs better for determining relationships between DNA matches?

A. It depends on what you’re trying to do. A centiMorgan (cM) measures the length of matching DNA segments. SNPs are a measure of the amount of information in matching DNA segments. Think of SNPs as the density of information times the length. So SNPs are better, right? Not necessarily. When you try to determine the genealogical relationship between two people, there’s an expected value for the length of matching chromosome segments. For example, you’d expect to share about a quarter of your cMs with one of your grandparents. Although, that value can vary (there’s a range it will fall in), there’s no reason that you wouldn’t have gotten segments from that grandparent that are particularly dense with information, which would result in more SNPs than expected. This compounds the uncertainty and thus extends the expected range of shared DNA if you’re looking at SNPs. So what are SNPs good for? They’re better at determining whether or not you’re related to someone at all. If you appear to share a segment with a DNA match, it could be identical by chance. It’s far more likely that an identical by chance segment occurs in a segment with low SNPs. It makes sense that it would be harder to randomly replicate a length of DNA code in a segment that’s more densely packed with DNA. Setting a minimum threshold for SNPs is the best way to determine that a person is a match. Then you can use cMs to try to determine the nature of that relationship.

Q. I found an unusually large number of DNA relatives on the same chromosome segment and I can’t seem to find a common ancestor between any of them.

A. Check to see if that chromosome segment is a known pileup region. Most of your DNA relatives on these segments will be related to you, but many generations farther back than what you’re looking for.

Q. I have a DNA relative who doesn’t share DNA with either of my parents. Why is that?

A. The DNA segment that you share with that person is probably identical by chance and it probably isn’t a very long (< 7 cM) or information dense segment (< 700 SNPs). It’s also possible that part of the match came from your dad’s homologue and the other part from your mom’s homologue, but neither of them shared enough DNA with the person to be considered a match. You could test this at GEDmatch by lowering the thresholds of cMs and SNPs and comparing your parents’ kits to the other person one at a time.

Q. Why do I have a DNA relative whose child shares slightly more DNA with me?

A. There’s probably a small portion at the beginning or end of the child’s segment that is identical by chance.

Q. Is it possible that I don’t share any DNA with one of my grandparents?

A. According to The Coop Lab, the chances of this happening are minuscule: You should have to go 7 generations back before there’s even a few percent chance that you have none of an ancestor’s DNA.

Q. My mother had a half-sister, both having different fathers. Can I assume that my mutual matches with her half-sister’s son are related to my maternal grandmother and that many of my DNA relatives who don’t match with him are related to my maternal grandfather?

A. You probably have about 25% of each of your maternal grandparents’ DNA and so does your mother’s half-sister’s son. You likely share about 12.5% of your DNA with each other. About 12.5% of the potential DNA matches for your grandmother should be mutual matches and about 12.5% should be related to you but not your half-cousin. Be careful not to mistake that second 12.5% for people who are from your maternal grandfather’s side. The remaining 75% of the potential matches for your grandmother won’t match with you at all.

Of your DNA relatives who match with you and not your half-cousin, about 57% are from your father, about 29% are from your maternal grandfather, and about 14% are from your maternal grandmother. For your DNA relatives whom you know are on your mother’s side, about 50% should be related to your grandfather and about 25% should be on your grandmother’s side but not related to your half-cousin. That means that ~25/75 = ~33% of your DNA relatives on your mother’s side would be incorrectly thought to be on your grandfather’s side if you assume that all of your maternal matches who don’t match with your half-cousin belong on that side.

It’s probably a safe bet that your mutual matches are from your maternal grandmother (check that all three match on the same segments), but most (~71%) of your matches who don’t match with your half-cousin and a third of those on your mother’s side will be from grandparents other than your maternal grandfather.

Q. Can I have anything other than 50% of my father’s DNA?

A. It should be 49–50%, depending on your gender. Chances are, if your test showed that you share 25% of his DNA, he’s actually your uncle, grandfather, or half-brother.

Q. I thought that humans and chimpanzees share 98.8% of their DNA. How do I have all of these DNA matches that share less than 1% DNA with me?

A. “Shared DNA,” in genetic genealogy terms, means long stretches of shared DNA that are half-identical. The “half-identical” part means that you share one of two homologues of a particular chromosome. You actually share 99.9 % of your DNA with all humans, no matter which homologue you compare. However, you have some very small differences in DNA with people. The length of a DNA segment before reaching any of those differences can be used to determine your relationship to a person.

Feel free to ask me about modeling & simulation, genetic genealogy, or genealogical research. And make sure to check out these ranges of shared DNA percentages or shared centiMorgans, which are the only published values that match peer-reviewed standard deviations. That model was also used to make a very accurate relationship prediction tool. Or, try a calculator that lets you find the amount of an ancestor’s DNA you have when combining multiple kits.

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