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Friends are as genetically similar as fourth cousins

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Re: Friends are as genetically similar as fourth cousins

#11

Post by LukeShu »

roninb wrote: post #3

In case you were also wondering what a "fourth cousin" was, someone who shares a pair of great-great-great grandparents[0]. Wikipedia has a great set of diagrams[1] in case you're also confused about first, second, or third cousins. I figure through induction you can presume any nth and nth removed from that page (I foolishly thought 3rd was where people arbitrarily stopped keeping count....) [0] https://www.referenc…

I used to really struggle with those types of diagrams. Like, they make sense on their own, but it's hard to reason about the patterns with them; the relationships (heh) between each diagram aren't clear.

I much prefer the square/diamond type diagram that's further down in the article. It seems like black magic at first, but if you stop and think about it for a bit, once you understand it, is crystal clear.

I guess it's about short-term or long-term ease of understanding.

Re: Friends are as genetically similar as fourth cousins

#12

Post by ams6110 »

roninb wrote: post #3

In case you were also wondering what a "fourth cousin" was, someone who shares a pair of great-great-great grandparents[0]. Wikipedia has a great set of diagrams[1] in case you're also confused about first, second, or third cousins. I figure through induction you can presume any nth and nth removed from that page (I foolishly thought 3rd was where people arbitrarily stopped keeping count....) [0] https://www.referenc…

Wow, takes me way back to an undergrad programming assignment to take a family tree and compute the degree and "removedness" of any two cousins.

Re: Friends are as genetically similar as fourth cousins

#13

Post by mnbcvx »

douche wrote: post #2

Where I grew up, it's a pretty good chance all my friends were fourth or fifth cousins. Small population + limited outbreeding = family hedges, instead of family trees. There were kids in my grade who couldn't date in town because all the girls were their first or second cousins...

> second cousins

Didn't stop my great grandparents :) My mom's side is Mennonite, which is also a very tight knit community. When my grandma sends random pictures from in and around Goshen, Indiana, my wife has commented multiple times "whoa that guy in the background really looks like you!"

Re: Friends are as genetically similar as fourth cousins

#15

Post by fiatjaf »

roninb wrote: post #3

In case you were also wondering what a "fourth cousin" was, someone who shares a pair of great-great-great grandparents[0]. Wikipedia has a great set of diagrams[1] in case you're also confused about first, second, or third cousins. I figure through induction you can presume any nth and nth removed from that page (I foolishly thought 3rd was where people arbitrarily stopped keeping count....) [0] https://www.referenc…

I get mad at people who think sons of first-cousins are second-cousins all the time -- and, frankly, almost everyone I know think this way.

I'm glad you linked to this Wikipedia article that proves I'm not mad into thinking there is a simple logic pattern that can be applied to cousinship.

However, this "once removed" notation is crazy. Why not second-uncle for parents of second-cousins and so on?

Re: Friends are as genetically similar as fourth cousins

#16

Post by runholm »

mnbcvx wrote: post #13
douche wrote: post #2

Where I grew up, it's a pretty good chance all my friends were fourth or fifth cousins. Small population + limited outbreeding = family hedges, instead of family trees. There were kids in my grade who couldn't date in town because all the girls were their first or second cousins...

> second cousins Didn't stop my great grandparents :) My mom's side is Mennonite, which is also a very tight knit community. When my grandma sends random pictures from in and around Goshen, Indiana, my wife has commented multiple times "whoa that guy in the background really looks like you!"

Really not an issue with second cousins. Second cousins are genetically different enough that issues with the children are just barely more common in non-related couples.

Re: Friends are as genetically similar as fourth cousins

#17

Post by aab0 »

ilaksh wrote: post #4

Could it just be more common for people who are in the same social group to be in the same ethnic group, and maybe that explains most of the genetic similarity they saw, rather than necessarily proving some kind of fitness advantage or something? Probably because often your friends are from the same neighborhood.

The genetic similarity here is estimated within the cohort; the Framingham cohort is by design ethnically homongeous to try to eliminate that sort of population structure confound. This way, the homophily is not picking up on the obvious stuff like ethnic groups. If they studied people from multiple ethnic groups, the results would be trivial and uninteresting. (This is why astazangasta's criticism in another comment is so amusing - this is meaningful because the sample is so 'biased'. This also holds for all the other studies done on Framingham.) But because only one highly homogeneous sample is studied, the chance genetic similarities of friends aren't due to just similar ethnic backgrounds, but are being caused by the genetic influence on things like SES and personality and intelligence and religiosity and hobbies and things like that. These similarities would probably also hold when considering a more unusual and cosmopolitan group, but it would be difficult to spot the increased similarity on a genome-wide basis because of all the racial differences in genomes (most of which would be nonfunctional and irrelevant to anything).

Re: Friends are as genetically similar as fourth cousins

#18

Post by mnbcvx »

runholm wrote: post #16
mnbcvx wrote: post #13

Earlier quoted context omitted.

> second cousins Didn't stop my great grandparents :) My mom's side is Mennonite, which is also a very tight knit community. When my grandma sends random pictures from in and around Goshen, Indiana, my wife has commented multiple times "whoa that guy in the background really looks like you!"

Really not an issue with second cousins. Second cousins are genetically different enough that issues with the children are just barely more common in non-related couples.

Yep. That's the kind of thing you look up at a young age when you discover you're the recent product of second cousins. Some interesting quotes from the "Cousin Marriage" Wikipedia article:

"According to Professor Robin Fox of Rutgers University, it is likely that 80% of all marriages in history may have been between second cousins or closer."

"Worldwide, more than 10% of marriages is between first and second cousins."

"In some countries it is seen as incestuous and is legally prohibited: it is banned in China and Taiwan, the majority of U.S. states, North Korea and South Korea."

https://en.wikipedia.org/wiki/Cousin_marriage

Re: Friends are as genetically similar as fourth cousins

#19

Post by dietrichepp »

fiatjaf wrote: post #15
roninb wrote: post #3

In case you were also wondering what a "fourth cousin" was, someone who shares a pair of great-great-great grandparents[0]. Wikipedia has a great set of diagrams[1] in case you're also confused about first, second, or third cousins. I figure through induction you can presume any nth and nth removed from that page (I foolishly thought 3rd was where people arbitrarily stopped keeping count....) [0] https://www.referenc…

I get mad at people who think sons of first-cousins are second-cousins all the time -- and, frankly, almost everyone I know think this way. I'm glad you linked to this Wikipedia article that proves I'm not mad into thinking there is a simple logic pattern that can be applied to cousinship. However, this "once removed" notation is crazy. Why not second-uncle for parents of second-cousins and so on?

Pardon me for my ignorance, but wouldn't sons of first cousins be second cousins?

Re: Friends are as genetically similar as fourth cousins

#20

Post by ironrabbit »

dietrichepp wrote: post #19
fiatjaf wrote: post #15

Earlier quoted context omitted.

I get mad at people who think sons of first-cousins are second-cousins all the time -- and, frankly, almost everyone I know think this way. I'm glad you linked to this Wikipedia article that proves I'm not mad into thinking there is a simple logic pattern that can be applied to cousinship. However, this "once removed" notation is crazy. Why not second-uncle for parents of second-cousins and so on?

Pardon me for my ignorance, but wouldn't sons of first cousins be second cousins?

Language quirk :) The son of my first cousin is my first cousin once removed. My son and my cousin's son are second cousins.
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