Is one ancestor's dna worth more than another's?

+7 votes
Is one ancestor's dna worth more than another's - in other words are you more like certain ancestor's because you have more of their dna?  Some thoughts...

1.  Direct paternal and maternal lines - since we are getting more information Y and mitochondrial from these lines -  does that mean that they influence us more than the other lines?

2.  The 50/50 split each generation isn't even - so some of the lines we have on paper technically might not show up in our DNA at all.  

3.  The X Chromosome - do the ancestors that contribute give us a little more than the others?  

Would love to hear educated responses (or uneducated but informed).  I don't know any answers but have wondered.
in The Tree House by Jonathan Wilson G2G6 Mach 1 (11.0k points)

It is my understanding that X is a given, males and females alike, and is in the ovum (the egg) and it it the sperm that contains the Y.  The last I read it was the combination of the two at conception that is required, and based on that fact, neither one is more valuable than the other. 

That Mr Jones might place great value on his Y-DNA and Mr Smith place an equal value on his mtDNA is a matter of individual preference, of their thinking.

Upon each strand of DNA, one each contributed from each bio parent, there are genes. These determine physical characteristics. Some have advanced the idea that they also determine the mental characteristics or psychological characteristics -- but without the physical neither the mental nor the psychological can exist, is what I think. 

And all the DNA test results indicate, or can indicate, as to ethnicity are probabilities. Likewise all those results can indicate is a probability that the maternal line is this and that the paternal line is that. And as we have seen, there are surprises when the probabilities indicate a NPE 

But the probability -- the chances, the odds -- are exactly that, they are not a guarantee of anything

Does Y, X and mt DNA influence us more than the rest of our DNA? Since it's an order of magnitude less info, it would seem not? But even if there is less overall influence, what traits this DNA does impart could have special meaning one's own life that could be highly influential. Science won't tell you because such a study would not only be subjective but mega multivariate.

I don't think the different DNA are ranked as to "influence".  

It is my understanding that DNA is a vehicle, and it is the genes riding on it that have the effects.  Whatever traits there are come with the genes, not directly from the DNA, because the DNA is a vehicle (it also carries more than the genes, if I recall what I read)  And my understanding might be faulty. 

Here's an article, might be a bit "heavy" but it does explain what DNA is and what it has What is DNA? - Genetics Home Reference - NIH

THIS ONE is a Beginner's primer, sparse but probably adequate in explanation DNA vs Gene: A Comparison For Beginners

8 Answers

+18 votes
Best answer

"Are you more like certain ancestor's because you have more of their dna?"

Quick answer on that one: Nope! Not really.

Before someone gasps and passes out because I tried to answer a question in six words...uh yeah no. It's more complicated than that, and you're cordially invited to skip the characteristically brief bit below--that was started over three hours ago before a conference call, but by gosh if I won't finish it even though there were great answers in the meantime.

And...I ran over the 12,000 character limit. So you get two posts for the price of one!

What we're "like"--which I'll define as meaning expressed phenotype--is all about coding genes, the little guys that are active in directing the production of RNA and proteins. And we're learning more and more that a lot of this is very complex activity, meaning the world isn't as Gregor Mendel saw it. It isn't a binary relationship of one gene, one expressed activity. In some cases it continues to appear to be (your hair color, for instance), but in most it's multiple genes influencing each other and their expressed output in a very intricate sort of dance.

It's not always straight-up "these genes do exactly this." What we are is actually a continual looping of physiological changes, epigenetics, and even our environment. A good example is a recent research paper I found interesting: "Adaptive Responses of Histone Modifications to Resistance Exercise in Human Skeletal Muscle." The part that's germane is that no fewer than 182 genes were found to have regulatory changes as a result of systematic, intense resistance exercise. The average increase in gene expression across 153 of these genes was almost 10%; another 29 genes saw suppressed expression.

As Michel mentioned, your DNA also changes as you age, regardless of epigenetic influences. This comes about with repeated cellular replication. In essence, the copy machine starts to wear out. The process is called deamination and is a result of DNA methylation. I'll avoid more TMI jargon, but these changes aren't just to body cells. The germline DNA can be affected, least in males. Males are basically on-demand gamete producers. We keep producing spermatozoa throughout most of our lifetimes, and that deamination process can result in changes to the DNA that's passed down to children. In the FTDNA yDNA surname projects, we saw experiential evidence of this years before sophisticated academic research showed us what was happening. Even with the nonrecombinant Y-chromosome, we saw that the older the father was at the time of conception, the more likely we'd see variances, mutations, in the STR counts.

Women? No methylation/deamination issues. That's because the female produces all her oocytes before she's even born. These are generally called the "primary oocyte" or "primordial follicle," and they go through most of the first stage of meiosis, including crossover (recombination), while the woman is still a fetus. So what the pituitary stimulates to completion with each menstrual cycle isn't new gamete creation--ergo no methylation. The number of oocytes she's born with is the total number of ready-to-go ova she can ever produce.

So in the big scheme of things, is the female's DNA "worth more" than the male's? Of course it is! (Just in case someone is reading over my shoulder.)

The female's DNA never gets altered or changed via methylation, but for genealogy it has a bit of a downside. You see, while male and female genomes are pretty much the same total size, about 3.2 billion base pairs (the X is a little larger than the Y, so the two Xs mean the female genome is slightly larger overall by about 2.5%), females undergo crossover, recombination, at meiosis a lot more than males. To the tune of about 70% more at each birth event.

And meiotic crossover is where we get the segments we use for autosomal DNA in genealogy. Numbers vary somewhat depending upon whom you consult, but as a baseline I stick with Harvard's David Reich: the female genome will undergo crossover about 45 times when producing an oocyte, and the male will do so about 26 times to produce a spermatozoa. The female genome will map out to about 4,780 total centiMorgans and the male to about 2,800cM (N.E. Morton, "Parameters of the Human Genome"). For genetic genealogy, you'll typically see 6,800cM used as the sex-averaged number for total autosomal DNA cMs in a genome (so minus Y and X chromosomes), which is how we get to 3,400cM as half-identical sharing with the parent/child relationship. But the values are all sex-averaged from the start.

by Edison Williams G2G6 Pilot (313k points)
selected by Jonathan Wilson

Part 2...

That's one of several reasons that the common measure for genetic relatedness, the centiMorgan, is very much a rough estimate, an interpolation of where we think those likely crossover points might be as based on a specific genomic map...and then we average the male and female results together. The cM is a handy guesstimate in genealogy, but it's far from accurate. Throw away reported decimal points and round to a whole number; when in doubt, round down. As a quick example using the GRCh37 map (now outdated by a few years, by the way, but it's still what all the autosomal DNA results we commonly see is based upon):

  • Chromosome 9
  • Position 29,216,761 to 70,574,578
  • Sex-averaged value: 11.68cM
  • Using female genome map positions: 20.53cM
  • Using male genome map positions: 2.83cM

So is that a meaningful segment to genealogy research or not?

All of the testing and reporting companies show sex-averaged values only. Which is realistically all they can do. Oh, and another point of confusion. The centiMorgan calculation is predictive, not historic. In other words, it estimates the probable likelihood of crossover occurring at a certain locus on a given chromosome during gamete creation. For a range of two values, we simply subtract the lowest cM value from the highest to get the segment values we're used to seeing. It is not an expression of how many cMs you inherited. May sound like splitting hairs, but the centiMorgan calculation doesn't, and can't, do that. Despite the fact that we're used to thinking of a segment of xxCm that we share with a cousin as representative of historic, generational inheritance, that isn't what the centiMorgan is. It's a simple interpolation equation using what we think will be the probable points of crossover. One centiMorgan is equal to a 1% chance a particular locus on a chromosome will be separated from a second locus due to crossover during meiosis. The calculation can't know whether your mother's gamete actually underwent 47 crossovers or 43; your dad's 28 or 24.

I'll leave off stuff about the Y-chromosome (very useful for genealogy along the strict patrilineal line; can be used for relatively recent generations as well as deep ancestry; has only about 72 coding genes in its 57 million base pairs and doesn't go through recombination, so a lot of room for distinctive variation and mutation; in aggregate, it's now believed that yDNA will see on average about one mutation every 2.7 generations) and mtDNA (not very useful for genealogy, but can be used to disprove a hypothetical matrilineal relationship; glacially slow for the germline to mutate, estimates are about one mutation every 70 generations (Andersen and Balding, "How Many Individuals Share a Mitochondrial Genome," PLoS Genetics; 2018); not much room to mutate because the tiny organelle's DNA is only 16,569 base pairs long and in there is a regulatory region and 37 genes that code for 13 polypeptides, 22 tRNAs, and two rRNAs, and some of these are life-or-death type genes; if you take all the known branches of the mitochondrial DNA haplotree and average them against the global population, you'll find that an mtDNA match is about as meaningful as saying you match someone because both of you live in San Diego, a city of about 1.4 million people).

Now that I've concisely left off any comment about uniparental DNA <cough> circling back to the beginning to wrap it up. In our DNA, humans are somewhere around 99.6% identical. Give or take, there are 5 or 6 million SNPs that will distinguish your genome from mine. The vast majority of that is not in the exome, the part of the genome that contains the coding genes, at least the ones that we know about. The term comes from "EXpressed regiON." The exome constitutes about 1.5% of your whole genome (which is why Whole Exome Sequencing is so much less expensive than Whole Genome Sequencing) and about 85% of all mutations there can mean deleterious health consequences.

The coding stuff is the baseline we start with to express our phenotype, "the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment." The exome is where the monetary value of DNA testing comes from. Big Pharma and others would like massive databases of Whole Exome Sequences with corresponding medical histories illustrating what those data may mean regarding health, wellness, proclivity towards diseases/syndromes, response to medications, etc.

Unfortunately for genealogy, our familiar, inexpensive, microarray DNA tests already began leaning that direction years ago. For example, 23.13% of all SNPs tested by Illumina's Global Screening Array v3.0 chip targets clinically relevant exomic DNA. So almost one-forth of all the results we get back from the chip are looking at loci in coding-region genes, not among the several million SNPs that are most useful for genealogy and population studies. And thanks to a phenomenon called genetic linkage, these exomic genes will stay in contiguous segments when inherited. The end result is why I always recommend researchers analyze their haplotypic pile-up regions as best they can before trying to build triangulation groups with small segments..."haplotypic" meaning your biologically ancestral haplotype as opposed to the few areas that have been found to be global-population level pile-ups. With the exonic region representing only 1.5% of our genomes but our DTC tests using 23% of the tested markers to explore that region, the exon is being heavily over-reported in proportion to genealogically meaningful SNPs. Over a fifth of the reported DNA from our common tests may be very unlikely to mutate for generation upon generation, and is a possible explanation why some see unusually "sticky" segments or, along some lines, relatively more DNA consistently inherited--or more correctly, reported--than should be explicable by the family tree. Genetic genealogist Debbie Kennett wrote a great blog post about this:

So... The most valuable DNA for genealogy? That which is thoroughly researched and rigorously analyzed.

Brilliant, Edison, as usual.  Now would you dumb that down into a few paragraphs for us dopes?

It is (still) my understanding you need both sperm and ovum and from that perspective neither one can be said to have more value, or to be worth more, than the other, whatever cogitation and perception an individual possesses may dictate ... 

laugh Edison, I'm never (yet) quite sure what you have imparted  even though I recognize most of the words and can google the others.  You may well bear the title of Lord Paramount of Explanations.  

What you say may be knowledge in that we can absorb and retain and use. There's parts of what you say where I have done this.

 Or it may merely be information like having all the complete collection of current yearbooks to the Encyclopedia Britannica we bought 30+ years ago (30 of them and another due to arrive) 

laughAnd in any case I'm pleased whenever you explicate. 

The advantage of a (much) more recent education. Well described Edison!
Never a quick answer, but always thorough and always worth reading, Edison!

C'est Bon Magnifique ! Edison !! 

Comment dites-vous, "way too many words"? But, you know: leopard, spots. I'm honestly better if I revise and edit a few times, but if I did that it would be like work.

A big thank-you, though, to anyone who even tried to read any of my ramblings in this topic. Jonathan's question was broad and open to essay-type answers. Exciting to me; dangerous to those around me.

+7 votes
Your question relates to the so-called 'nature-nurture' debate in biology. See an intro here:

It  really is nearly impossible to say what part of your inherited DNA 'contributes more'. DNA is not a strict thing, you can have changes in DNA during life as well, being passed on. Hence the need of having an error margin on top of all those percentages given by many (eg stating on is 27% German is clearly unscientific).

If it relates to building your tree: Some parts of your genome are more conservative, and therefor are better indicators for your lineage. The Y chromosome is a strange one, and the mitochondrial one as well.

Taking the word 'worth' literal: DNA with specific links to diseases/deceases is worth much more ($) than plain DNA.

Tip: Startpage will tell you if I am Educated enough to give this answer.
by Michel Vorenhout G2G6 Pilot (223k points)
edited by Michel Vorenhout
+10 votes
The Y chromosome has very few genes, and most of those are concerned with sperm production. Your (biological) father was by definition fertile, so that's not very informative. The Y also has a gene SRY that turns on a cascade of genes that are active during embryonic development (e.g. increasing testosterone levels).

mtDNA codes for genes involved in energy metabolism, a universal and essential housekeeping function. Again, this is not a very "interesting" set of genes in terms of traits. A person in haplogroup H will function the same as a person in haplogroup J or A or L or what have you. There are mutations that interfere with its function, however.

The X chromosome has a very large number of genes. Mutations in some of these genes do influence traits (e.g. color vision). Males have only one X, so they will express whatever traits are on that copy. Females have two X's, and frequently one copy can compensate for a mutation on the other X.
by Ann Turner G2G6 Mach 1 (10.6k points)
+7 votes

Because autosomal, Y, and mtDNA require separate tests, it is useful to decide on your purpose before buying a test.  For most purposes, autosomal will be most useful.  You will have matches from both sides of your family.  You would use Y-DNA for tracing your paternal line (the Wilsons, most likely, unless you were adopted, or in certain other circumstances).  For tracing your direct maternal line, you might want to take an mtDNA test (although I have some doubts about whether you might get any useful results).

Over time, it is true that DNA from some of your distant ancestors will disappear; i.e. you will not inherit any.  If you were to create complete chromosome maps for yourself, you might observe that.  Or more likely, you could see how even in a couple of generations you might lose one grandparent's DNA on one particular chromosome, etc.

In my own experience, X-DNA is harder to work with and thus less reliable.

Edited upon rereading question and to clarify.
by Julie Kelts G2G6 Pilot (428k points)
edited by Julie Kelts
+3 votes

Thanks for all the information!  Let me see if I am understanding you all correctly.  

1.  Direct paternal and maternal lines - since we are getting more information Y and mitochondrial from these lines -  does that mean that they influence us more than the other lines? 

No - out of 64 possible 4th great grandparents it is possible that I am getting just as much of my phenotype from any one of them - not just the one whose last name I bear.

2.  The 50/50 split each generation isn't even - so some of the lines we have on paper technically might not show up in our DNA at all.  

Yes.  I might have a legitimate paper trail to a 4th great grandparent with no matches at all because their dna was simply not passed to me.   

3.  The X Chromosome - do the ancestors that contribute give us a little more than the others? 

Yes.  But not much more than sex. 

by Jonathan Wilson G2G6 Mach 1 (11.0k points)

Jonathan, one can affirm item 1. and 2.  

As to 3.  I think from what I read, my understanding is that the X study results would narrow the range of possible matches for blood kin. 

And the basic use of any DNA study in genealogy is to hopefully support the evidence of the paper trail, and in general it quite often does.  The X study itself merely narrows the number of matches one might have. 

The word "influence"? The word if I understand the definition is the capacity to effect the character, behavior, or development.  In the case of DNA and the genes, that "influence" is a given, it's built in, it's inherent, it is from conception onward 

Since the mtDNA and the Y-DNA and the X so closely tied to them are all part of the one and same DNA strand, they come in a package, they are packaged together, it is science and popular Media that has caused the perception you could pick and choose as if shopping at a store but the fact is that you take one the others come right along attached as ever they were 

And as has been stated, living your life will affect the package of DNA / genes you inherited at conception and it is the life you live that has a great deal of influence ... sets of diploid twins who "lead different lives" (have different life styles) reflect this effect of living life on the DNA package

Let me try:

1. 'just as much' or 'just as little'. It really is not a 1 to 1 case. You will get 'some', but 'some' might be very much shared with all the others, so who knows who contributes more.

2. Nope. If these people are biologically connected, there is some genome of them in you. It is more a question of: can you prove it? You can only take DNA from recent people, not from them (unless you are very famous) So you need known descendants that are willing to share their DNA tests with you.

3. The X chromosome is one that males (XY) and females (XX) have. You get your sexe from having or not having a Y chromosome.

Ok I think 1 & 3 are settled.

But there seems to still be a discrepancy on #2.  

My understanding was that with each generation 50% is passed from the father and 50% from the mother (roughly) however within that 50% from one parent it is more random which 50% is passed.  So for example Mom passes to me 50% of my genes, however of those genes she might pass to me 70% from Grandma and 30% from Grandpa.  

The result of this is that the further generations we go back the more diluted the genes are from that ancestor.  Which is why a paternity test can tell for certain if someone is not someone's father (no 50%) but that certainty is lost the further you go back.

A very concrete example would be my mother's DNA test which shows many matches that don't show up on mine at all.  We have the exact same geneaology (on her side) but I cannot find matches to certain ancestors because she did not pass them to me.

Yes, there is for sure a chance so strictly you are right about 2 (see edit below).

But: it really depends on the line and the type of test. For the maternal/female line you should get help from the Mitochondrial DNA, for the male from the Y chromosome (unless there are some more advances now).

Add/edit: also note that for genealogy you need to have markers, parts of the DNA that is variable and unique. So you are really talking about inheritage ("their DNA) of those markers (first part of your statement), not of DNA itself. Apes and humans share over 99. something% of the DNA, it is the differences that you are looking for.

(So: use papertrails ;) I am for sure not advocating at all to do DNA tests).

Jonathan, regarding your question 2, here is an example that might help.  I have attached my chromosome map to my profile.  You can see that on chromosomes 16, 17, 18, 21, and 22, for either my paternal chromosome, or my maternal, or both (on chromosome 22), there are no crossovers (recombinations).  So, for example, on chromosome 16, all my maternal DNA comes from my mother's father.  Because of that, anyone she matched maternally on chromosome 16 would not match me.

(The method, more or less, for creating this kind of map is described by Blaine Bettinger here.)

Jonathan, on item #2, I think Michel recognized the two factors at work that can get confusing. For genealogy, there's a distinct difference between all the DNA you inherited, and what DNA we can actually detect with our current, common testing methods.

Here are two links that might help. There's obviously some jargon in both, but you can swiftly get to the relevant stuff. The first is from well-known population geneticist at U.C. Davis, Graham Coop (I'm referencing his publicly published material, so I think I can use his name). This is a November 2013 post titled "How Much of Your Genome Do You Inherit from a Particular Ancestor?" It isn't a long post, and what he's looking at is a modeling of the autosomal DNA you get, patrilineal and matrilineal, generation over generation. What he calls "blocks" we'd typically call "segments." The quick-look chart is one that asks, "What's the probability I inherited zero blocks of DNA from any given ancestor?":

There's another chart immediately below that one that considers individual chromosomes. But the aggregate shows that the odds are pretty darned good that you get some DNA from every ancestor back through your 3g-grandparents. Starts to decrease with your 4g-grandparents, and by your 6g-grandparents the odds are starting to bomb out quickly. BTW, you'll also note that the chart illustrates what I tried to describe earlier about a female's DNA going through crossover more frequently than a male's, so the zero-blocks-of-DNA probability curve is steeper for the matrilineal line.

But... Nobody is going around exhuming ancestors to test their autosomal DNA. So the fact that we might have inherited some DNA from 7g-grandfather Seamus doesn't mean we can detect that we did. We can only test contemporary relatives, and each generation farther back adds unique meiosis events and unique genomes. That's why, looking at theoretical average sharing, you and your 1st cousin will match-up on about 12.5% of your DNA; you and your 2nd cousin on about 3.125%; and a 4th cousin on only about 0.195%. And, technically, the odds of any two ova or spermatozoa being completely autosomally identical are one in 222 (or 4,194,304:1); so there's ample variety to go around even among siblings and even if there was a bit of pedigree collapse in the past.

In 2012, the then chief scientific officer for 23andMe, Brenna Henn, and six colleagues published a cryptically titled paper, "Cryptic Distant Relatives Are Common in Both Isolated and Cosmopolitan Genetic Samples." I refer back to this study frequently, because a portion of it is directly relevant to genetic genealogy.

They used data from 23andMe and the Human Genome Diversity Panel to look at, in part, determining "bounds for predicted degrees of relationship given the amount of genomic IBD sharing in both endogamous and 'unrelated' population samples." There's a lot of good stuff in the paper, but maybe the single most pertinent chart is this one (click on it for a full-sized version):

Based on detectability, while we may be carrying DNA from 7g-grandfather Seamus, we have only a 0.24% chance of being able to detect it...based on the fact that we have to test cousins to find it. We'll leave revered grandpa Seamus undisturbed.

Now that morass I made of phenotype and genealogy rears its head again. Remember all that rambling about coding genes and stuff? One reason that the exome makes up only about 1.5% of your genome is that coding genes are, on average, pretty gosh-darned small. Let's take a specific hypothetical.

Assume that 7g-grandfather Seamus was proudly Scottish and had red hair. Red hair is a recessive trait caused by a few mutations in a gene referred to as "MC1R" (melanocortin 1 receptor). The gene is located on chromosome 16 from position 89,918,862 to 89,920, overwhelming 2,110 base pairs long. Our microarray tests could be programmed to look for a couple of those specific, mutated base pairs, but as a segment MC1R would never show up on our test results. Way too small.

However, hair color is an obvious part of our phenotype. Very identifiable...well, unless dyed or until we go gray. We all remember how dominant and recessive genes work; Mendel's wrinkled peas and all. Someone with black hair can still carry the recessive gene on the other of his chromosome 16s, and pass it down to future generations. In Scotland, for example, the estimate is that fully 40% of the population carry at least one chromosome 16 copy of the red hair mutations, but only about 13% of the population is redheaded.

The upshot being that revered 7g-grandpa Seamus could have been passing down his red-hair mutations via the tiny MC1R for many generations until it got to you. If your other parent also had the recessive mutation, you could be a redhead yourself. Or you might have dark hair but still pass along the red hair mutation to your children.

Even if Whole Genome Sequencing becomes the norm, you may never be able to trace that the recessive gene for scarlet Scottish locks actually came from Seamus. That might be the only minuscule bit of DNA that survived through all the generations to be bestowed upon you. But it's potentially a very recognizable one.

P.S. Thanks for the best answer star. I'm trying to make up, a little, for that unfocused ramble here.

Edison, now I'm confused.  Are you saying that if Jonathan has red hair, he could have got it from his seventh great grandfather Seamus, but inherited nothing else from Seamus?  I can see how that might happen, but it still has to be part of a segment he inherited from one parent or the other, doesn't it?  And similarly, going back to Seamus?  Isn't all the DNA we inherit, detectable or not, contained within the segments that we can detect?
@Edison, you are really good at explaining it. On using a name for citation: you have to. It's bad (real bad) if you would not have done so. We are not discussing his family links but his/her work.


Yes, the red hair could have been present in Seamus. And yes, that might have been the only block of DNA you will be able to detect from Seamus (I am not using inherited here as that word is much wider).

And yes, you only get DNA from your biological parents (and then all the changes in your life), so yes, one of your parents.

Then your last question: kindof yes. It does not mean the companies can detect everything. It also does not mean one detects all inherited DNA. As far as I know, all tests still use the microarrays Edison mentions. Those use pre-programmed (known) sequences of DNA. If those couple with a part of the DNA from the sample, you get a hit. If you are unable to pre-program the sequence ( a block, or marker as I called it earlier) you will never know.

If you would use a company that does full sequencing you would have the data overkill problem: you would need to use a library to search for markers, basically the same problem.

Not sure if this makes things clear though ...
Michel (and Edison):  I have created chromosome maps for myself, as I described above, and have identified all my crossover points except one.  Thus, nearly all my chromosomes are completely accounted for by the segments I have identified, and there is nowhere else for the autosomal DNA to be except within those segments.

In theory, I believe it is possible to identify earlier segments by comparing my maps with various of my matches.  It would be remarkable to be able to get back as far as a seventh great grandparent.  I do see how a tiny segment with nothing but the red hair DNA could be part of one of those larger segments.  It would have to be.

Thanks, Michel. I do explain...meaning I can go on and on forever. But I've never become good at explaining well.

Julie, sorry I didn't have time yesterday evening to get back to your questions. Michel did a good--and more succinct--job in my place. Now I'm going to ramble a while in order to talk about my blatant tendency to ramble.

I know my word count in DNA discussions is flat-out stupid. And I happily make fun of myself about that. In 2013 I saw a quotation on the FTDNA Forums that read: "Brevity, especially when it comes to something as operationally complex as the interpretation of autosomal DNA, comes at the expense of accuracy. I prefer accuracy." I even stuck that on my genealogy website.

Because, fact is, genetics--however we genealogists may wish otherwise--is a pretty darned complex subject. There's a massive amount we still don't know, and new things are being discovered, technologies developed, on almost a monthly basis. We've moved from 19th century wrinkled peas to completion of the Human Genome Project in 2003. Then we realized that the HGP had a lot of errors and omissions. Its goal, after all, was an accuracy rate of no more than one error in 10K base pairs...and the smallest segment our microarray technology can realistically detect is over 300 times that size. But molecular biologists from Harvard and U.C. Santa Cruz have estimated that as much as 8% to 9% of that sequencing was faulty.

We've gone through 32 revisions of the human genome reference sequence map since then, and some feel that the genome is too cluttered and complex even to be represented by literal mapping the way we're currently doing it. In an opinion piece in Genome Biology in August 2019, The authors note, in part:

"...The allelic diversity within the reference genome is not an average of the global population (or any population), but rather contains long stretches that are highly specific to one individual. Of the 20 donors the reference was meant to sample from, 70% of the sequence was obtained from a single sample."

We're still even questioning the foundations of what we think we know about DNA. As more and more discoveries are being made via long-read, nanopore, and other sequencing technologies, the clearer it becomes that DNA is far more complex than we thought. Says Antony Jose, professor of cell biology and molecular genetics at the University of Maryland, "DNA cannot be seen as the 'blueprint' for life. It is at best an overlapping and potentially scrambled list of ingredients that is used differently by different cells at different times." (Antony M. Jose, "A Framework for Parsing Heritable Information," Journal of the Royal Society; 22 April 2020. Sergio Pistoi, "DNA Is Not a Blueprint," Scientific American; 6 February 2020.)

Okay. I'm getting all esoteric. At least for now, none of that seems to impact our use of autosomal tools when it comes to fairly close cousinship comparisons. But when we start moving beyond that range of about 3rd cousins, some of what we have decided is accurate, have taken as "common knowledge," has no scientific foundation; some of the voices we listen to who write and speak on the topic of genetic genealogy have no academic or professional background at all in biology, biochemistry, or genetics.

An example is autosomal triangulation. I subscribe to more daily email updates from academic publishers and research organizations than I ever have time to scan. I even "publish" an online newspaper that does automated aggregation to search the web daily for what might (keyword "might"; it pulls in some odd stuff occasionally) be interesting research: And to my knowledge there still, here in August 2020, has never been a single peer-reviewed study that put the concept of ancestral identification via autosomal DNA triangulation to the test. It may be a valid methodology; it may not. There are biological reasons that would weigh against its being accurate. It may be valid for a recent few generations, while invalid or suspect for deeper relationships. We think it works (at least for those recent generations) because we see it produce results we expect it to produce.

But that's kinda the very definition of confirmation bias.

Here's another example. I used this once before and, while I'm infamous for my World's Worst MetaphorsTM, this may be the very worst of all time: DNA is not a roll of toilet paper.

A ramification of common-practice genetic genealogy has, seemingly, led to a supposition that inherited chromosomal segments are like equal-sized, perforated sheets on a roll of toilet paper. That because we got these 45 sheets from mom, what we pass on will be one or more of those neatly perforated sheets. Ditto backwards through all our great-greats.

A well-known genealogist has written that your ancestral segments "are fixed between fixed crossover points created when your parent passed these chromosomes to you."

Thing is, your DNA doesn't know anything about who your ancestors are and which bits of DNA they passed on to you. The statement indicates that crossover locations are hardwired, are perforations in that roll of toilet paper, determined not by the biological mechanisms of meiosis but solely by your eight great-grandparents, your 128 5g-grandparents, and so on. If that statement were true, we'd never be able to calculate centiMorgans at all...even if our current calculations are probably only grossly accurate. The centiMorgan is an interpolation calculation using that "standard" human genome sequence map: a 1% probability of crossover equates to 1cM. If an individual's crossover points are hardwired based only on what each of their ancestors contributed, our centiMorgan calculations would be unique and very different from person to person.

This is one of those biological factors that makes deep-generation autosomal triangulation questionable. All your DNA has to come from your 128 5g-grandparents. But in total they passed down about 4,544 segments to get you that DNA. How those segments recombined in each successive generation (averaging about 35 newly-demarcated segments per ancestor) isn't hardwired. It's primarily controlled by three factors during meiosis: crossing over, independent assortment, and genetic linkage (the tendency of base pairs that are close together on a chromosome to be kept together during crossover). For the latter, keep in mind that genes are small lil' suckers. The largest we know about is 2.3 million base pairs; and in the realm of genes, that's positively Brobdingnagian.


--Part 2--

The average size of a human gene is between 10K and 15K base pairs. That's why the whole of the exome--that area of the genome that contains the coding genes we've currently identified--makes up only 1.5% of your 3.2 billion base pairs. In areas away from the genes, genetic linkage has, generally, less impact. So there are large regions of your genome where crossover points can be more random.

And it isn't always a simple swapping mechanism at crossing over. When chromosomes rearrange, other things can happen, specifically deletions, duplications, inversions, and translocations. Some of those are in response to a need to do some DNA handyman work, some chromosomal repair. That's why we have two versions of each chromosome, and why over half of the Y-chromosome is full of repetitive stuff that the Big Y test doesn't even bother sequencing.

When genealogists think of DNA segments I believe they tend to visualize some physical manifestation they can easily grasp, like a puzzle piece or an index card...or a perforated piece of toilet paper. But that's perhaps a kind of dissociative metaphor. Heck, we also operate with the elegant image in mind of a symmetrical, color-coded, double-helix with graceful steps up the "ladder." If you look at DNA under an electron microscope, you might not even be able to guess what you're seeing:

That's the centromeric region of a human chromosome. It's actually messy in there, isn't it?

These are some of the reasons, I'm sorry to say, that you haven't identified all your crossover points...and never will. You can identify, roughly, the segments that you inherited from your parents, your grandparents, your great-grandparents. I say "roughly" because, keep in mind, our microarray tests look at only about 0.02% of your DNA. That's like examining one square inch of your front lawn to determine what the whole yard is like. So not only are centiMorgan values only general estimates, but the start and stop points reported for segments aren't precise, either. Based on the average coverage, if you have a segment that's 8 million base pairs long (so broadly approximate to 8cM) you're seeing only 1,625 milestones among the 8 million. And some of our reported segments have far lower SNP density than that; GEDmatch defaults to a sliding 200-400 SNP minimum. All that can be reported is the first milestone actually tested, and the last. A contributing factor as to why different testing companies will sometimes report very different segments for what seem like the same thing.

And of course not to mention that of the 0.02% of your genome tested, the microarray chips in use by different companies can be looking at as few as 20% of the same SNPs.

All of these are reasons why we don't get equal amounts of DNA from each grandparent, from each great-grandparent. Why DNA from some biological ancestors vanishes in the course of generations and some--maybe like Seamus's mutations for red hair--keep on lingering. Extremely small blocks of DNA are transmissible. Our microarray tests just can't identify them, and WGS testing doesn't yet have the IT infrastructure and tools to let us compare for ourselves. Using again that very broad average of SNP coverage in our microarray tests--and keep in mind that up to 23% of those SNPs may be targeting medically-relevant SNPs that really don't affect genealogy--we really can't effectively detect segments smaller than about 2.5cM to 3cM. In the Henn, et. al. study I linked to before, they determined that a region had to be at least 5cM to be considered in the study; their determination was that their computational modeling method was accurate down to 7cM.

I use WAY too many words because a lot of what is propagated about DNA use for genealogy is just flat-out unsubstantiated. Genetics isn't a simple A+B=C world. I use a lot of the common-place tools and methods myself...but I like to believe I'm rigorous in their application and that I have a modicum of understanding about their constraints and limitations. For example, the deepest autosomal triangulation I've been willing to accept was to a 4C1R. For that I have 19 test takers from six separate branches of that tree; the shared segment was analyzed for coding gene content (if the gene density is high, the likelihood of genealogical relevance is low); it was compared against known global-population pile-ups; haplotypic pile-up charts for each of the six branches were constructed and compared to determine if the segment was an artifact that might be too old to accurately correlate; and where available the kits were run through GEDmatch's triangulation and one-or-both tools to see if it might be possible to find indication the segment could have any possible inheritance path other than the one shown in the detailed family tree.

We simply can't make multi-generational genetic genealogy quick, easy, and concise. Because it isn't.

Edited: Becasue Because I am a typo machine, I tell ya! A machine! And thanks to Kay Wilson for spotting a rather, er, egregious one.

Thank you Ed for your sharing your knowledge with us. I am glad to see someone else questioning the practice of autosomal DNA triangulation. This is something I've been wondering about for many years now because it just didn't make any sense to me for anything other than as a method for assigning segments to specific ancestors based on matches with close relatives. I wrote two blog posts on the subject back in 2016 which include links to the relevant scientific literature:

In the second post I discussed what I considered to be some of the alternative explanations for the phenomenon of multiple people matching on the same segment, all of which seem far more likely to me than having multiple people all matching on the same segment by virtue of descent from a single recent common ancestor. I don't have any background in genetics but I do at least try and read all the relevant scientific literature. One of the basic principles of IBD detection is that it is not just the amount of DNA  shared that is important but also haplotype frequency. The more people who share a segment the less likely it is to be of recent origin. It seems to me that triangulation should be used to try and find sections of DNA which are only shared with one other cousin rather than sections shared by multiple cousins which is what I see people doing at present.

We seem to have a lot of genealogists who are unfamiliar with the concept of uncertainty and always want to find proof. I also find it odd that some of the people who promote triangulation seem to be much less interested in using clusters, yet this technique has been validated for forensic use for identification purposes.  

(Waving at Debbie) But, for those who don't know you, you're representing yourself way too...lightly. I think a research associate in the Department of Genetics, Evolution and Environment at University College London, Education Ambassador for the International Society of Genetic Genealogy, and a co-founder of the ISOGG Wiki (and I can confirm its primary contributor) qualifies in our circles as a background in genetics. Plus, two published books dealing with genealogy, a writer on DNA for all the UK's major family history magazines, and a frequent and sought-after speaker at conventions and conferences, including the massive "Who Do You Think You Are? Live" conference.

(Note: I clearly start rambling to the world at large often...well, almost all the time. But because I might first address someone by name, it can seem I'm still speaking to that specific person. So here's notification that I'm now climbing on my well-worn soapbox on the street corner; just the crazy man talking to any and all passersby.)

I really want autosomal triangulation to be a valid methodology. I really do. But the concept originated with the non-recombinant Y-chromosome, and I think it was merely extrapolated from there as, "Hey! This should work for autosomal DNA, too." Which is comparing apples and oranges. The STRs and SNPs we test on the Y only modify via mutation; other than the PAR they're untouched by meiotic crossover and independent assortment.

There are a number of factors I believe we can point to as hypotheses why autosomal triangulation shouldn't be consistently accurate beyond the most recent generations and, frankly other than experiential--not experimental--findings (usually gleaned from an individual's own family research, which perforce means that he or she is seeing only very isolated samplings, only narrow haplotypic ranges), not much evidence to indicate that triangulation can be consistently accurate.

One critical factor that I believe is often overlooked is precisely what Debbie described in Part 2 of her 2016 blog post on the subject: basic probability. We talk a lot about how much DNA sharing we can/should expect between test-taking cousins. And we all know that the probable expected sharing drops precipitously with each step in cousinship, by a factor of 4 at each full "C," a multiplier of 0.25. That's simply because each distance in full cousinship introduces additional meiosis events in order to make the DNA trip from Test-Taker One, back to the most recent common ancestor, and forward in time again to Test-Taker Two.

With triangulation, it isn't about how much DNA you would be expected to share with that MRCA, or how much with a test-taking cousin. In order for triangulation to be valid, all the test-takers in a triangulation group must share some significant amount of the exact same DNA from that MRCA. So just like rolling dice, the odds go up significantly with each die you include. The odds of rolling two dice and having them match are 1 in 6. You roll three dice, the matching odds increase to 1 in 36.

Unlike dice, meiosis doesn't represent strictly independently-random events. There's a method to the randomization, and structural differences in the chromosomes...even between some male and female crossover "hotspots." I built a table once that used only the coefficient of relationship to try to illustrate what the probabilities looked like for three (and another column, four) distant cousins all sharing some of the same DNA as a distant common ancestor. And I decided going only by the CoR wouldn't work, that I lacked the knowledge to come close to an accurate calculation. Suffice to say, though, that like continually adding another die into the roll, that the odds against the same-same DNA outcome increase dramatically with each test-taking cousin you add to the mix.

The chart Debbie provided us from AncestryDNA shows this. At the 3rd cousin level, Ancestry indicates that any three matches can be expected to share a meaningful portion of the same segment(s) from a given ancestor about 14% of the time. Meaning that, by Ancestry's data, triangulations among three 3rd cousins should only be possible once in every 7.14 attempts. For three 4th cousins, it's a fraction over 1%: you would need 99 triangulation attempts to find three 4th cousins who match on the same segment that came from a specific one of the 16 3g-grandparents.

I don't believe we'll get a solid answer--or any answer--on autosomal triangulation until we have a rigorous research study that can use NGS testing, not our common microarrays, and that can find a way to accurately compare generations of descendants to the actual results of one or more distant ancestors. My hope there is that this may come from exhumation samples obtained from notable, historic individuals.

But I believe it's extremely difficult, if not in some instances impractical, to perform genealogical triangulations to distant cousins using our inexpensive microarray tests. The problem is that, from day one, these chips have never exclusively tested any specific subset of the 5 million or so SNPs that would be most genealogically informative. In fact, a past president of the Open Genomes Foundation told me some time ago that when Illumina introduced the first OmniExpress chip, consideration was given to samplings in the exome, but that the remainder of the SNPs selected were pretty much random, with the concern being evenly-spaced genomic coverage more than any particular ancestral relevance. I can't confirm that, but as early as v1.2 of the OmniExpress-24 chip we already saw (if we look at the specs for sampling of RefSeq exons, ADME genes, SNPs in the Gene Ontology dataset, etc.) that a little over 20% of the SNPs targeted were in the coding-gene, exome region. Starting with the GSA chip, though, Illumina gave us nice illustrations of the breakdown of tested SNPs, this one of the GSA v3.0 chip:

Clinically-relevant SNPs comprise about 20% of all markers tested by the default GSA v3.0 configuration. Further, the company specifies that 262,173 intronic--the genomic area we'd most be concerned with for genealogy--markers are included, so 40% of the 654,027 markers. But even the intronic markers targeted aren't necessarily the most valuable ones for ancestral genetics.

Bottom line here is that I believe we see large numbers of reported triangulations that can't stand up to scrutiny. I think one of the first steps in triangulation needs to be to see if any meaningful portion of the mutually-shared segment(s) is in the exome, if it includes multiple known coding genes. As an absurd example, that neither I nor my cousin is lactose intolerant isn't genealogically very informative; our ancestors probably developed that mutation as they moved into northern Europe before the last ice age. But trying to eliminate exonic SNPs from triangulations is not that easy to do. If we have loci detail, we can at least refer to the GRCh37.p13 version of the 1,000 Genomes Browser to explore the segment manually, but I don't know many who include that in their process.

Second step, for me, would be comparison to haplotypic pile-up regions. We don't yet know about many global-population level pile-ups, so that lookup is fast and easy. Since many times we're comparing matches against cousins who have tested at different companies and/or different microarray chip versions, I think it helpful to try--if the data can be accessed--to compile haplotypic pile-up charts at different companies. Ancestry tries to winnow out some of these for us with their Timber algorithm, but we don't get to see any of the detail there. For more about haplotypic pile-ups, see this January 2018 post by Debbie.

Third, my wish-list includes being able to understand the SNP continuity in a segment. Various reports give the SNP density, how many SNPs were used in the comparison, but today we can't get a report of the actual continuity and positions of those SNPs. In other words, is the proclaimed segment comprised of a stretch that's a virtual SNP desert with most of the SNPs clustered in one or two narrow bands? Are the SNPs matching in a mostly one-after-the-other sequence, or is the mismatch allowance algorithm permitting a relatively high percentage of SNPs in the overall segment length that don't match (e.g., ignoring one or two mismatches every 50 may be permissible to the reporting utility)? This has become more of an issue with the introduction of the GSA chip and the lack of same-SNP overlap from previous tests. That's why GEDmatch had to walk-back their earlier matching defaults and move to a less rigorous threshold of a floating 200-400 matching SNPs, and why MyHeritage introduced imputation in the form of what they call "stitching": using genotype modeled information to "fill-in" missing gaps from disparate microarray data to predict two otherwise very small segments might actually be one modest one. But it's still guesswork.

Ultimately, hopefully, one day we can start using whole genome sequencing data for genealogy and rely less on predictive genotyping. Back when I did my first yDNA test in 2003, we were ecstatic when the 37-marker panel came along to help refine our results. A 37-marker match was a lot more solid than a 12-marker. And the more yDNA testing has progressed, the more refined those matches have become. However, some folks coming from the world of autosomal testing first tend to expect more markers tested equals more matches. It's just the reverse, though. At 12 markers I might match 5% of the whole of the UK. But with the Big Y test I match a dozen men in the genealogical timeframe of the 36 in our Williams subproject, most of those specifically recruited to participate.

I think the same is going to be true of the stage we're at with autosomal triangulation: the more research advances and the more hard data we get about our genomes, the more likely that most of the distant-cousin triangulations we see now will be negated. The biology and the probabilities just aren't on our side.

A simple question.  What is the difference between triangulation and clusters?  I see MyHeritage flag certain matches of a match as “triangulating” and others as not.  My understanding of clusters on GedMatch is that it includes all of the related to both in a cluster and marks those that triangulate with a triangle.  

So then the more interesting question, why do you prefer clusters over triangulation?
+2 votes
Thank you all!  This is quite an education into how dna genealogy works (or doesn't).  

So one more follow-up question?  Is genetic genealogy at this stage a pseudo-science like phrenology?  Or is it an immature science, like much of medicine or psychology in the early 19th century?  

Are we going to be laughed at by genealogists of the future for believing that the "matches" we claim as cousins now as science discovers that although there is correlation it really has little to do with common ancestry.  

Or are we going to be hailed as pioneers who blazed trails that later genetic genealogists will travel at blinding speeds and far greater accuracy as our scientific tools catch up with the desire to know our origins?
by Jonathan Wilson G2G6 Mach 1 (11.0k points)
I would say immature as a science and thus a pseudo-science when presented to you by Companies that claim they can tell you the truth while basically (yeah yeah) they want yet another DNA sample for their library.

On the other hand: genealogy has been practiced for centuries, generating very neat books with or without any source mentioned. Does that make it a science or merely a descriptive branch of research (or applied science maybe)? So does genetic genealogy need to be a science?

Jonathan, I've singlehandedly made your topic run a few thousand words longer than necessary. Sorry 'bout that. But can you tell that, if I'm in the mood, I do enjoy these kinds of broad, open-ended, essay questions?

And Michel is doing a great job beating me back to the topic and summarizing my thoughts much better I do myself. But, heck yeah: Trail Blazers!

The life sciences are squirrelier than math and the physical sciences. We might be able to envision a future where the unified field theory is a done deal, where we know precisely what quantum mechanics had been trying to tell us all along, and where biology isn't as...singularly squishy anymore. Where we've learned enough about it that we can apply binary yes/no operations and know what it will do.

However, we aren't there yet...ergo the recent academic opinion articles about whether or not we even have DNA described in a fundamentally correct way. What we understand about genetics has been on a track that's as fast as has been all the computer-based technological marvels we've seen the past several decades. I had dinner with James Watson and Francis Crick once (well, it wasn't like a personal sit-down; it was a university departmental thing). That's actually what first sparked my interest in DNA. They were engaging speakers at the time and, even well over a decade after their Nobel Prize, the subject so energized them, and the audience, that it seemed almost like science fiction.

We've gone from the first use of electrophoresis in 1952 to give us any sort of image of DNA structure at all, to the first DNA sequencing in 1977 (of a tiny bacteriophage), to the very first use of microarrays to test DNA in 1995, to the pretty astounding advances in whole genome sequencing within the past five years. Not to mention that as of this summer, across all platforms, about 35 million individuals have taken at-home DNA tests.

The fact, though, is that genealogy isn't big on the scientific research radar. For universities, nobody gets grants to research genealogy tech; population genetics and anthropological DNA, yes. With corporate funding, the big bucks are in medical and pharmacological research. That Brenna Henn paper I mentioned is one of the very few scientific, peer-reviewed studies in the past several years that specifically addressed ancestral identification...and all the researchers involved worked for 23andMe at the time and had access to their database. The company has been less fiscally interested in genealogy research in recent years.

IMHO, where we are is a point at which the basic application of DNA to genealogy is science. With autosomal DNA we can clearly see consistent and accurate application to identify recent relationships, back to at least 2nd cousins. All the work of the Adoption Angels and forensic operations like Parabon and the work Cece Moore does wouldn't be adequately predictable if the very basics weren't right.

But what's happened is that we've extrapolated basic methods that can fairly easily be confirmed (e.g., a lot easier to validate the result showing a living 2nd cousin than to exhume select ancestors to see if the hypothesis still holds true back to 7g-grandfather Seamus...or 15g-grandfather Connor) and we've presented them as established methodology, as science-backed genealogy.

And that's the part that is, as Michel notes, pseudo-science. Some of what we're doing right now with genetic genealogy may prove to be totally flawed. Some of it may be groundbreaking even if arrived at only by intuition.

The problem we've got is that, once again, genetics is a complex and continuously developing field. But on the genealogy side of that we have ardent, intelligent genealogists who want to use--and very rightly so--DNA to augment their research. I for one believe it's the most significant addition to the toolbox since the notion of online-available documentary sources.

The companies who provide DNA testing have something to sell. They want you to believe your decision to donate your lederhosen and buy a kilt is solid science; they want to show you a whole boatload of matches with predictions out to the nth cousinship because it adds value and attractiveness to the consumer experience...even if there is nothing in the scientific literature to strongly support it.

And we have numerous websites and voices actively teaching genealogists that using DNA to trace back to your 7th-great Seamus is simple and easy. What you'll find, not unexpectedly, is that the voices who have graduate degrees in a biological field are not the ones preaching that DNA is easy or that we have a body of scientific knowledge that's yet adequate to fully understand the possibility and accuracy of correlating the revered Seamus via small-sample microarray tests to his living 8th cousins.

I've been awfully curmudgeonly in these posts. I'm not at all dissing the use of DNA for genealogy. Shoot; it's part of my life. But, the Dunning-Kruger effect: there are an awful lot of folks telling us how to use genetics in genealogy who really don't have the background to do so. I, again, don't mean working with closer relationships like the Adoption Angels do, or WikiTree's policy for "Confirmed with DNA" out to 3rd cousins. I mean things like the assumptive leap that if identifying 3rd cousins via DNA is easy, then identifying great-Seamus should be just as easy. That's the pseudo-science. A+B=C, therefore A+B+C must equal D.

There's good science in play in genetic genealogy. And there are good resources to learn more about the fundamentals of genetics. But if something looks like it might be unfounded, poke at it with a stick, a lot, until you can figure out if it's science or pseudo-science.

Edited: Gasp. I actually shortened it. Some...

Good post, Edison.  Very clear, if not brief (but I guess I should have seen the original).  smiley

+1 vote
There is a lot of good comments already existing; but, let me add this:

Y-DNA is easiest to follow because it involves only one family line. It can reach back thousands of years. The limitation is that if the direct male line is broken, the father had no sons, that is as far as you can go.

mtDNA involves many families where obtaining tree info is more of a problem. It can reach back thousands of years. The same limitation exist if a mother has no daughters.

The auDNA only reaches back 5 generations across all ancestors and the percentage of segments inherited can differ each generation.

The x is provided by both the parents if it is a girl and only from the mother if a boy.

This is my understanding so far.
by Dale Ladnier G2G6 (7.0k points)
Dale, autosomal DNA goes back, and can be traced, farther than five generations, although it gets more difficult and less reliable.
+2 votes
Thank you all for the great discussion.  Thank you Susan, Mike, Edison, Michel, Julie, Pip, Gerald, Ann and Dale.  

Happy hunting!  

by Jonathan Wilson G2G6 Mach 1 (11.0k points)

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