Comments on: What is inbreeding?

By: Gary

Gary — Sun, 29 Jul 2012 19:47:10 +0000

Sorry if this has already been covered, but it appears that Willard Mitt Romney is pretty well inbred, given that his grandfather was with the Mormons in exile in Mexico for practicing polygomy. My understanding is that he had 5 wives.
The actual history of this religion turns my stomach, and it’s no wonder they were driven out of Ohio, then Missouri, then Illinois, then Utah into Mexico. Considering how cruel they were to their own women and girls, not to mention everyone who was not of their cultish ‘faith’, it it truly amazing that anyone would associate themselves with present-day Mormons.
My 5th grandparents were wiped out in the 1850′s by the Mormons in the Mountain Meadow Massacre, along with more than 120 other men, women, and children. Google this piece of history for another look at Mormon history. Romney as president makes my skin crawl.

By: Grey

Grey — Thu, 26 Jul 2012 00:39:12 +0000

“William Henry Harrison was the product of 2nd cousin marriage.”

I think the question will turn out to be more one of either
a) consciously repeated close-kin marriages among the same group of families for alliance or inheritance reasons
or
b) unconsciously repeated close-kin marriages among the same group of families simply due to a *very* low total breeding population.

I doubt one-offs among an otherwise outbred population are likely to matter much unless particularly unlucky.

If so the Icelandic situation where you have a small population breeding exogamously where that small population is just large enough to avoid the second case may turn out to work very well.

By: Solis

Solis — Tue, 24 Jul 2012 22:14:05 +0000

William Henry Harrison was the product of 2nd cousin marriage.

By: Ed

Ed — Tue, 24 Jul 2012 17:51:34 +0000

@Razib

Sorry if this is late, and forgive any misunderstandings I might have.

So basically hybrid vigor vs. inbreeding yeah?

How deleterious are the effects of inbreeding caused by population bottlenecks?
Would you consider this study evidence of hybrid vigor like some other posters were in the comments?
http://dienekes.blogspot.com/2012/03/african-american-athletes-have-more.html

By: Grey

Grey — Tue, 24 Jul 2012 05:10:38 +0000

“Some of the fear of inbreeding is more like a leftover from old traditional superstitions and old wives tales, without much connection to actual genetics.”

I wonder. I think it might depend on how quickly things like IQ inbreeding depression could lift. Say you had a bunch of inbred villages in a valley and a new settlement is created in virgin forest and settled by individuals from each of the local inbred villages who then inter-marry. Would the grand-children of the settlers already have changed enough to see their relatives in the inbred villages as some kind of mutant?

(I mean mutant in comic book terms as inbred people are always portrayed that way in western or at least Anglo western culture. I wonder how old that portrayal is?)

By: Isabel

Isabel — Tue, 24 Jul 2012 03:42:51 +0000

“the medieval population bottleneck was a major issue. i don’t think they’re that troubled in regards to recessive diseases, so it goes to show that the real problem is 1st cousin marriage, and repeated instances. though i guess i could do some simulations to see….”

Some ancestors are overly represented in Quebec’s current population, resulting in modern occurrences of recessive disease, including several forms of ataxia, even though the Catholic church required dispensations for marriages closer than 4th cousin. They did give out a few, but as far as I understand it, the appearances of the particular diseases in some French Canadien populations is due to the alleles spreading through drift as the population grew rapidly after the original bottleneck. Eventually the recessive, mutated allele was so widespread that people carrying it would marry and have affected children even if they were not knowingly related.

By: Razib Khan

Razib Khan — Tue, 24 Jul 2012 02:18:54 +0000

selection clears away the poorer variants very quickly.

i don’t think there’s evidence of a lower number of deleterious in icelanders, but i could be wrong (this is a well studied population).

By: Tom Bri

Tom Bri — Tue, 24 Jul 2012 02:06:50 +0000

Modern Icelandics benefit from the rough climate and the poverty of their ancestors. If in a family of six or ten children all but two die, selection clears away the poorer variants very quickly.

By: John Emerson

John Emerson — Mon, 23 Jul 2012 20:28:06 +0000

I went through an Austrian Habsburg line once and found that one Emperor has 16 different ancestors at the 64-ancestor level.

That’s some pretty determined inbreeding.

Some inlaws of mine fear a family curse based on an uncle-niece marriage 2 or 3 generations back. There’s insanity in the family but I think that it’s more likely the result of several generations of concentrated meanness, nastiness, suspicion, and grudges.

/ inlaw-bashing.

Some of the fear of inbreeding is more like a leftover from old traditional superstitions and old wives tales, without much connection to actual genetics.

By: AndrewV

AndrewV — Mon, 23 Jul 2012 20:12:30 +0000

I’ve put up a bunch of posts relating to inbreeding recently

And failed to mention in my opinion, one of the best articles I have ever come across on the subject, even from just a historical and human interest perspective:

Inbreeding & the downfall of the Spanish Hapsburgs
http://blogs.discovermagazine.com/gnxp/2009/04/inbreeding-the-downfall-of-the-spanish-hapsburgs/

By: Razib Khan

Razib Khan — Mon, 23 Jul 2012 19:56:15 +0000

#12 & #13, they are like jews, but even more so. everyone in iceland is related on the order of 2nd or 3rd cousins i think:

http://scienceblogs.com/gnxp/2009/01/16/genetic-variation-in-space-tim/

the medieval population bottleneck was a major issue. i don’t think they’re that troubled in regards to recessive diseases, so it goes to show that the real problem is 1st cousin marriage, and repeated instances. though i guess i could do some simulations to see….

By: AndrewV

AndrewV — Mon, 23 Jul 2012 19:55:19 +0000

@ 12 and 13

http://newsfeed.time.com/2012/02/09/icelanders-avoid-inbreeding-through-online-incest-database/

a search engine called Íslendingabók (the Book of Icelanders) allows users to plug in their own name alongside that of a prospective mate, determining any familial overlap. The site claims to track 1,200 years of genealogical information about the island’s inhabitants.

The database may be found here :
http://www.islendingabok.is/English.jsp

However:

The database is in Icelandic and is unfortunately not available in other languages.

Access to the genealogical database Íslendingabók is currently limited to Icelandic citizens and legal residents of Iceland who have been issued an Icelandic ID number (kennitala).

By: John Emerson

John Emerson — Mon, 23 Jul 2012 19:44:11 +0000

To me it’s peculiar because in Iceland it seems quite likely that some people have exactly the same ancestors 10 or 20 generations back even though their more recent ancestors don’t overlap at all. This could be mathematically presented as though it were serious inbreeding (identical gene pools), even though I think that that would be erroneous.

By: John Emerson

John Emerson — Mon, 23 Jul 2012 19:38:46 +0000

What is the Icelandic situation? As I understand, almost all Icelanders are descended from a few thousand early settlers up to about 950 AD. (435 are named, but presumably the list had a big bias toward males, freemen, and landowners). All or almost all were Norse or Irish. But I don’t know how much replenishment there has been since then, and I know that there’s been more than one bottleneck since. After 950 AD Iceland was never so rich as to attract immigrants, but presumably there was replenishment after the die-offs.

By: nextgenseek

nextgenseek — Mon, 23 Jul 2012 18:37:14 +0000

interesting post on inbreeding. Here is a practical definition, at least for mice. Jackson Labs, which sells inbred mice defines inbreeding as
“”
An inbred strain is one that:

is produced using at least 20 consecutive generations of sister x brother or parent x offspring matings, or;
is traceable to a single ancestral pair in the 20th or subsequent generation.
“”
(http://research.jax.org/grs/type/inbred/index.html)

By: Hassan

Hassan — Mon, 23 Jul 2012 18:32:05 +0000

Another example is John Adams who married his third cousin Abigail.

John Quincy Adams, the 6th President, was the product of this marriage.

By: Chad

Chad — Mon, 23 Jul 2012 15:54:26 +0000

FDR was the product of 6th cousin marriages, otherwise I don’t know of any.

By: Razib Khan

Razib Khan — Mon, 23 Jul 2012 14:49:21 +0000

In this sense

that’s the problem right. we have to say “in this sense….”

#6, yes, obama surely has the fewest runs of homozygosity. as for the converse, there are many examples of ethnically homogeneous presidents. i would bet john adams or van buren. anyone know if a president was the product of cousin marriage?

By: rich lawler

rich lawler — Mon, 23 Jul 2012 13:47:48 +0000

Interesting discussion…indeed inbreeding is slippery as it can be calculated using individuals, for example by quantifying the inbreeding coefficient using Path analysis (counting up the number of lines of descent that an allele has from ancestor to descendant)–this is akin to what Krishna’s grandmother has suggested (in the comment above), and one can calculate the average inbreeding coefficient in a population, where we keep track of different individual inbreeding coefficients and take their average–this value, usually denoted as F, is what is then plugged into equations to calculate expected genotype frequencies under inbreeding (assuming H-W conditions). As Razib noted, inbreeding doesn’t much change allele frequencies, only rearranges the probability of getting homozygotes. There is also the idea of random versus nonrandom inbreeding, since in a small, closed populations there is likely to be some amount of random inbreeding simply by chance (e.g., if every potential mate in this small population was a 3rd cousin). Nonrandom inbreeding is the amount of inbreeding above this random (or chance) level; for example in the population of 3rd cousins, some folks explicitly seek out 1st cousins as mates. In this sense, inbreeding is a microevolutionary phenomenon, in that most researchers are only concerned with the properties of the population a few generations back, and the degree to which the background population is related as well as the degree to any two individuals in the population choose mates who are more (or less) related than the background relatedness. Too much inbreeding (choosing mates who are more related than the background relatedness) can lead to inbreeding depression, which is the term for the negative consequences of inbreeding. This distinction–inbreeding versus inbreeding depression–arises since not all inbreeding is generally bad (at least theoretically). Fisher suggested that a sister can gain a benefit by mating with her brother and also mating with a random male. Her direct fitness gain would be N (# of offspring) if she mated with the random male and she would also gain indirect fitness gains (r*N) by improving her brother’s mating success by mating with him. So her total fitness is (1+r)*N, whereas it would only be N if she mated with the random male.

A nice introduction to inbreeding is given in John Relethford’s new book “Human Population Genetics.” This book is basically a gentle introduction to pop-gen, with some human examples thrown in.

By: Christopher@BorderWars

Christopher@BorderWars — Mon, 23 Jul 2012 13:06:39 +0000

So it’s a safe bet that Barack Obama is our least inbred President, given that it’s unlikely any common ancestor could be found between his father’s Kenyan Luo ancestry and his mother’s mixed European ancestry. Any consanguinity between his sets of grandparents would be rendered nil by that great genetic distance. He’s quite possibly an outcross tens of thousands of years in the making.

If I had to venture a guess on the Presidents most likely to be inbred, I’d suggest the first 10 or so Presidents would be the most fruitful given the demographics of the colonies and the isolation of money and power.