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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

#91

Post by logfromblammo »

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…

Through induction, you can also say that your sibling is your nilth (0th) cousin, and your identical twin is your negative-first cousin. That makes aunt or uncle a nilth cousin once removed, and a great aunt or great uncle a nilth cousin twice removed. Your parents are negative-first cousins once removed.

If this sounds ridiculous, that's because it is, as a natural consequence of the ridiculous nomenclature.

It would be far less confusing to just insert an unordered pair of numbers for both individuals' genetic distance from their common ancestor. So instead of a first cousin once removed, you would have a (2,3)-cousin. Identical twins would be (0,0), parents would be (0,1), ordinary siblings (1,1), aunts and uncles (1,2), and so on. A fourth cousin would be (5,5).

But that doesn't account for half-siblings, so it would be inadequate for some family relationships.

Re: Friends are as genetically similar as fourth cousins

#92

Post by logfromblammo »

davnicwil wrote: post #76
avodonosov wrote: post #65

Earlier quoted context omitted.

Arbitrary? You have the same parents with sibling; with cousin you share only one of the two pairs of grandparents.

Actually that's often but not always true - brother b1 and sister s1 from family 1, with parents g1 brother b2 and sister s2 from family 2, with parents g2 s1 has child x with b2 - x has grandparents g1, g2 s2 has child y with b1 - y has grandparents g1, g2 x and y are cousins, and also share the same 2 pairs of grandparents :-)

Those are "double first cousins". I even have some triple cousins in my tree, where three siblings from one family married three siblings from another family.

Re: Friends are as genetically similar as fourth cousins

#93

Post by Noseshine »

Manishearth wrote: post #89
Noseshine wrote: post #59

Earlier quoted context omitted.

> Given that the US is a lot more racially heterogenous The US? Sure, but this isn't about the US. The population chosen (or available) for the study does not reflect that diversity. https://www.framinghamheartstudy.org/participants/index.php > The Original Cohort of the Framingham Heart Study consisted of 5,209 respondents > of a random sample of 2/3 of the adult population of Framingham, Massachusetts So I don't un…

I used to live in Malden, and IIRC Framingham and the other suburbs were pretty diverse. That's what I was basing my comment on. Of course, my personal bubble could have skewed this -- looking it up it seems like Malden is more diverse than Framingham, but Framingham seems to still be close enough to the overall distribution in the US. Not the same of course. shrug > The study isn't "wrong" of course, it clearly stat…

Okay, here is what you wrote:

    > Given that the US is a lot more racially heterogenous than the majority of the world
"The US". You didn't say "Framingham". You know, it's really a PITA to have discussions with someone who can't just say "I made a mistake, I edited my comment" or something like that and instead try anything else.

Re: Friends are as genetically similar as fourth cousins

#94

Post by logfromblammo »

mod wrote: post #27
Throwaway23412 wrote: post #25

Earlier quoted context omitted.

Edit: Nvm. I am more confused than I thought.

I've always wondered about this one: My father's first-cousin-once-removed is what to me?

You are a (1,0)-child to your father (who is a (0,1)-parent to you). Your father is either (2,3)-cousin or (3,2)-cousin to the other person. It is ambiguous from the terminology. So you are either (3,3)-cousin or (4,2)-cousin to them. That is second cousin for the former and first cousin twice removed for the latter.

Re: Friends are as genetically similar as fourth cousins

#95

Post by thaumasiotes »

jhbadger wrote: post #71
thaumasiotes wrote: post #50

Earlier quoted context omitted.

What jacobolus said. First cousins are also genetically different enough that issues with the children are just barely more common than in unrelated couples. (The relative risk adjustment is quite high, but the absolute risk is still low.) If marrying your first cousin is a culturally accepted practice, it happens all the time and people get much more inbred than you might expect from looking at, or calculating from,…

But first-cousin marriages generation after generation are not a great idea, as happened in various royal houses of Europe in order to make sure both parents were of noble blood. In fact, it has been estimated that the last Hapsburg ruler of Spain, Charles II, ended up with more homozygous alleles than someone who was the offspring of siblings would. And he had numerous physical and mental problems that were probably…

Your reply starts with "but", but I don't see the contrast with anything I said? My comment specifically notes that cousin marriage as a recurring practice causes much bigger problems than a single case of cousin marriage in isolation. That note is more than half of the entire comment.

Fun fact: people like to present Charles II as the inevitable result of ongoing inbreeding, but he was noticeably less inbred than more competent historical rulers. Amenhotep I and his great wife had, between them, two parents, two grandparents, and two great-grandparents. Unsurprisingly, he and his great wife turned out not to produce any viable children. Unsurprisingly, that line also experienced early death outside of combat. But surprisingly, they don't seem to have been plagued by mental difficulties at all; as best we can tell from the record, they were very strong kings.

Re: Friends are as genetically similar as fourth cousins

#96

Post by [deleted] »

davnicwil wrote: post #76
avodonosov wrote: post #65

Earlier quoted context omitted.

Arbitrary? You have the same parents with sibling; with cousin you share only one of the two pairs of grandparents.

Actually that's often but not always true - brother b1 and sister s1 from family 1, with parents g1 brother b2 and sister s2 from family 2, with parents g2 s1 has child x with b2 - x has grandparents g1, g2 s2 has child y with b1 - y has grandparents g1, g2 x and y are cousins, and also share the same 2 pairs of grandparents :-)

[deleted]

Re: Friends are as genetically similar as fourth cousins

#97

Post by auganov »

bzbarsky wrote: post #88
auganov wrote: post #72

Earlier quoted context omitted.

Such a rate of childhood death would be high even for sibling parents. So perhaps there was something else going on.

> Such a rate of childhood death would be high even for sibling parents. Really? In the mid-19th century? https://ourworldindata.org/child-mortality/ says "in 18th century Sweden every third child died, and in 19th century Germany every second child died" (that's before the age of 5; the usual definition of child mortality). The "England and Wales" chart on that site shows that in the mid-1800s child mortality was ab…

Thanks, didn't at all realize how bad it used to be :O Even today it seems to be much worse than I'd naively assume.

Re: Friends are as genetically similar as fourth cousins

#99

Post by jccalhoun »

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?

yes. the wording and the fact that other people seemed to agree with the op made me spend way too much time on this.

Re: Friends are as genetically similar as fourth cousins

#100

Post by amelius »

thesz wrote: post #82

Related: https://www.sciencedaily.com/releases/2008/02/080207140855.h... Third cousins are most fertile. So, marrying friend is a good idea from fertility point of view.

"Fertility" does not necessarily mean "fit offspring", but your point stands.
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