How closely related are two random British people on average?

I heard if you take two random people, they are almost certainly (97.2% chance) at least your 16th cousin. However there’s only about a 1% chance they are your 11th cousin! :eek:

I’d imagine it would be closer for the same ethnicity on average, especially an ethnicity with a fairly small population (for example, the Amish or Latvians). As an American of British-Irish descent how closely would I be related to say Emma Watson, Cheryl Cole or the Beatles? Maybe around the 12th cousin range or so?

I think 12th cousin is a good ballpark estimate. Of course you’d be 13th or 14th cousins in a variety of ways, and so on, so your net consanguinity would be closer than 12th cousin, even with 12th cousin the closest single connection. (I think I knew how to estimate that net consanguinity once, but I’m abstaining from coffee for medical reasons and thus have no chance of performing calculations. :frowning: )

Interesting. I wonder if Europeans and Japanese are still related in any way - I suppose they could share a distant Mongol ancestor or something perhaps.

Is there a more scientific (or I guess more measurable) description of “relatedness” than “Nth cousin”? Is it possible with genetic testing and such to come up with a number say where 0.0 is the minimum “relatedness” of any two human beings and 1.0 is the “relatedness” of identical twins.

I can’t find an easy-to-use data table for this, but this one study looks at genetic diversity within nations and its effects on their economies:

Page 30 has a graphic showing diversity by region & country… but I can’t translate that into understandable familial relationships.

(Note: This is way out of my field, and I don’t understand it. But it is going on.)


There’ also the wonderful book Seven Daughters of Eve that traces human evolution and migration through the mutations of mitochondrial DNA in a very easy to understand manner and it discusses how human populations diverge, isolate, mix, etc. over time and how it can all be traced with their genetics.

I’m glad that no one has brought up West Virginia yet.

If you’re just looking at it at the family tree level, rather than with genetic testing, then the number you’re looking for is consanguinity, as mentioned by septimus. And finding “net consanguinity” when multiple relationships exist is easy: You just find the consanguinity of each relationship and add them up. For instance, Frodo and Bilbo Baggins, who are “second and third cousins, once removed each way” have a consanguinity of 1/64 (for second cousins once removed) plus 1/256 (for third cousins once removed), or 5/256.

Of course they are-- They’re both human. Heck, even if they weren’t, all living things on Earth are related somehow or another.

I’m sure I’ve heard b4 that 1/3 of europeans are descended from Genghis Khan. I don’t know if that 1/3 extends to the British Isles. But it would leave many europeans very closely related and 1/12 cousin is possibly even exaggerating our distances.

100% of Europeans are descended from Genghis Khan. The same is likely to be true of any random goatherder you can think of contemporary to Genghis. What’s remarkable about Genghis is that 8% of the population of his former empire, or half a percent of the entire world, is descended from him in the entirely male line. That is to say, those peoples’ father’s father’s father’s … father’s father was Genghis.

I know that they made an app in Iceland where you check if the person who you are having sex with is not your cousin.

This is spectacularly false. Perhaps you are thinking of the identical ancestor point for Europeans. That states that, if you go back to some point in time, everyone alive was either an ancestor of everyone today or an ancestor of no one. I think you forget about the second part.

The only way 100% of europeans could be descended from genghis kahn would be if the other 2/3 had procreated with the 1/3 who were descended from him. It’s unlikely that would have happened since it probably took a long time for the 1/3 that’s descended from him to build up. They’d only have reached that current number (if it’s true) by procreating with others who weren’t descended from him.

If someone has a very rare allele, there is a 50% (1/2) chance that his/her sibling will share the allele. (More if the parents were closely related to each other, but never mind that.) A 1st cousin would have a 1/8 chance of sharing the allele; 2nd cousin 1/32 chance; and so on. Replace that chance § with (1 - log2§) to get the “degree of relationship”. Twelfth-cousins have degree 26, or .000003% (1 in 33 Million) chance of rare allele sharing.

I think Chronos is correct that these probabilities are simply added for independent cousinships. In a complex genealogy with multi-way ancestor sharing it gets more complicated. If you Google for such calculations, I think you’ll find websites for dog breeders! (Breeders want purity but not excessive inbreeding, so perhaps tune matings carefully.)

One can approach the average consanguinity calculation from the other direction. If Britain had 33 Million unrelated inhabitants at some point in the past, had random matings thereafter, no immigration, and “smooth” vital statistics, then after centuries of mixing the rare allele sharing probability would obviously be 1 in 33 Million: the same consanguinity as 12th cousins. But of course those assumptions are all faulty.

Figuring 25 years per generation, 12th cousins have a common ancestor born 325 years before them, say in the mid 1600’s. I know less than 2% of my 17th century ancestors, but even so can see that Winston Churchill and I are “certainly” 13th cousins 2x-removed, using a 16th century connection. (This was an unbiased experiment; I’d never checked for my consanguinity with him until a moment ago.) Based on this sample of one, I might agree that 11th cousin (24 degree relationship) would be a better guess than 12th to OP’s question. “Net consanguinity” would knock that down a few degrees.
BTW, Genghis Khan was born in the 12th century and, since many of the Mongol Khans had astounding procreative success, he surely has a huge number of descendants including, probably, almost all Russians. Relatively few firm connections show up in published West European pedigrees, AFAICT, but of course that doesn’t mean they don’t exist.

That’s why I said that it’s likely to be true of the random goatherd. Not everyone from that time has left any descendants at all, but most people have.

Not only are Europeans and the Yamato people (the dominant ethnicity in the Japanese Home Islands) related, the degree of genetic consanguity is consistent with a division of at most about three millenia, which is substantially less than that between many geographically separated ethnic groups in subSaharan Africa.

While 100% of people of historical European heritage are almost certainly related to Genghis Khan to a degree of genetic consanguity greater than, say, Acamapichtli or Labibela, it would be logistically improbable for them all to be descendents, and essentially impossible to quantitatively assess the absolute degree of consanguity to Khan or any other single individual except on the patrilineal side owing to the variability of mitochondrial DNA associated with the progengy of Genghis Khan and his descendants.

It is a relatively trivial exercise in probability to look compare the ratio of combinations versus permutations stemming from an original set of N pairs of independent parents with M numbers of offspring over G generations to get a flavor for how much chromosomic variability is possible and what the mean degree of genetic consanguity is likely to be (assuming a uniform distribution in which variations don’t affect rates of reproduction on any given line). In reality, for any population some chromosomal lines will tend to dominate for both social and physiological reasons (notwithstanding external influences such as genocide, plague, and other factors that disproportionately target particular lines) which means that average variability is significantly less than the uniform distribution, and of course in any real world populations there will be geographically isolated clusters in which certain lines will be heavily consanguious while others are entirely excluded.

Stranger

Both the Indo-European-speaking Hittites and Minoans developed advanced civilizations well before 1000BC. It is reasonable to assume that linguistic affinity at that time denoted genetic affinity, placing the Hittites and Minoans within the peoples historically considered to be European. If so, then the split between Europeans and Yamato must have occurred much earlier than 1000BC, and I think there is likely to be other evidence against such a recent split as well.

Someone once calculated for me that seventh cousins were exactly the same relationship to a person as the AVERAGE other person in the real gene pool. My original question was 'How excited should you be to find out you are an nth cousin- when does it make no more sense to say you are relations than your relationship to a random person in your nation.

Of course it depends on your heritage- I have no non-British ancestors despite the fact that I am 5 to 25 generations back on various lines.

It’s fun to perform a sanity check on this with some fairly basic assumptions and arithmetic.

First cousins have a common ancestor two generations back. At two generations back, you have 2^2 forebears. For an n-th cousin, you go back n+1 generations and have, at a maximum, 2^(n+1) forebears. You may have less because there may be duplicates, but you can’t have more.

Therefore, when comparing family trees to see if you are n-th cousins with somebody, you compare your pools of 2^(n+1) forebears to see if there are any matches.

Assume that these forebears are chosen from a closed population of N. Therefore if you are not n-th cousins or closer, every single person from Pool #2 is not a member of Pool #1.

Assuming randomness, and letting 2^(n+1) = p, the chances of any given member of Pool #2 not being a member of Pool #1 are:

P = (N-p)/N
= 1-p/N

And therefore the chance of every member of Pool #2 not being a member of Pool #1 is P^p.

So the probability you are not n-th cousins is P^p = (1 - p/N)^p

We can use the first exponential assumption to approximate:
(1 - p/N)^p = 1 - (p^2)/N

Substituting back for p, the probability that there is no overlap between the sets is therefore:
1 - [4^(n+1)]/N

And therefore, the probability that there is overlap is:
[4^(n+1)]/N

The OP asked about 11th and 16th cousins, so that’s 13 and 18 generations, respectively. Roughly 15 generations. Taking an average of 25 years per generation, that’s roughly 400 years, or roughly AD 1600. Let’s call the global population in 1600 = 650-million

So for 11th cousins, that’s
[4^(11+1)]/650-million
= 16.8-million / 650-million
= 2.6%

And for 16th cousins it should be 2^5 = 32 times greater, or about 83%.

So the OP’s numbers aren’t completely out to lunch.

Note that as n increases, there will be more overlap within each pool; intra-pool overlap means that these numbers are maxima.

If I’m understanding you correctly here, you’re saying that a typical modern European will probably not have any genetic material derived from Genghis. I’m not referring to that, but to descent: That is, he’s probably one of my great[sup]N[/sup] grandfathers, even if (by the luck of the meiosis draw) I haven’t inherited any of his genes.