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Comment by mikeash | Hacker News Reader

The action of recessive genes is well understood, as are various diseases caused by them.

Don't forget that European royalty conducted a long-running experiment on the effects of inbreeding, and the result was a great deal of deformities and disabilities. A whole genetic disorder is named this way: just look up "Habsburg jaw." Lots of other experiments on human inbreeding have been run on islands like the one in the story.

Your post sounds an awful lot like, "I don't understand, therefore it is not understood."

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More like no one would ever write about the need for Jews to outbreed, and if they did they would be rebuffed with a bazillion qualifications about the science being uncertain. You only get to make sweeping statements when politics are on your side.

The Jews are several orders of magnitude more numerous than these small island populations, and even then there are genetic diseases that are far more common among them.

So since the science of recessive genes is so well understood, how big does the population need to be before scientists instruct the group to start outbreeding?

Scientists typically don't outright instruct, but merely describe the consequences of various options. Sometimes (e.g. with global warming and the "keep dumping massive amounts of CO2 into the atmosphere" option) the consequences are "lots of people die horribly." But outright instructions are typically left up to the nice men with large guns.

My point was that the quantity of the effect matters. If the science can't quantify how bad having a population of 150 is, then we can't say we understand things well enough to recommend "outbreeding", as the original post did.

Why do you think it can't be quantified?

Let me put it another way: Can you or anyone else provide scientific studies that would directly back the claim

The level of inbreeding on that island is quite scary. Although if they purged a lot of the deleterious alleles from the population, if they outbred with others, they could produce much healthier offspring and a lower likelihood of various genetic ailments.

And not the general statements that have been made in this thread, but something that would pin down a specific numeric relationship between population size in humans, and health.

EDIT: and for reference, the prevalence of Asthma may be purely due to the founder effect, not an effect of inbreeding per se. Two or three of the original settlers suffered from asthma[0].

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2222.1974....

http://www.nature.com/nature/journal/v397/n6717/abs/397344a0...

Rougly 1.5 new deleterious mutations per diploid genome per generation. If you have closely related individuals having children, there is a greater chance that the deleterious mutations will overlap.

But the question is how the population size relates to health, which you haven't answered here. You have numbers for the number of deleterious mutations, but that doesn't directly give a quantitative relationship between population size and health.

It's because your question isn't really properly formed. What do you mean by "health"? The chance of a person having a really shitty genetic disorder? The fitness cost of inbreeding as a function of coefficient of relationship? The probability that a bottlenecked population will go to zero due to harmful mutations, as a function of the bottleneck size?

I would be interested in seeing any of these quantified. And I'm not asking for people to educate me, but rather for people to take a more careful look at which claims can be directly justified by the science, vs which claims are assertions made based on extrapolating from very different data.

The claim was that the population size of Tristan da Cunha is problematic. I still haven't seen any direct evidence presented of this fact.

> The claim was that the population size of Tristan da Cunha is problematic. I still haven't seen any direct evidence presented of this fact.

You linked to an article about the higher incidence of asthma on that island yourself (above), and the reason why the number of individuals is so high directly traces to the islanders' gene pool.

However, I wouldn't say the population size is unsustainable. They are more likely to be decimated by a random disease or, over generations, by a shared mutation. But the population is not nonviable. The issue is more that, to the modern humanitarian mind, they will have to deal with more ill effects due to the genetic monoculture.

These might help. Biological fitness is definitely quantifiable and is a major component of population genetics.

http://en.wikipedia.org/wiki/Genetic_purging

http://en.wikipedia.org/wiki/Inbreeding_depression

I'm having trouble understanding this line of reasoning:

> purely due to the founder effect, not an effect of inbreeding per se

Genetic bottlenecks greatly amplify the effects of any genetic abnormalities for obvious reasons. It sounds like you assume inbreeding and genetic scarcity are two different things when they're not.

Yes, the founders brought these genes into the population. That is the whole point when we're making observations based on the inbreeding which caused a higher incidence of symptoms as compared to the mainland human population - exactly because there is a high chance for recessive traits to be expressed.

If you mean inbreeding does not necessarily lead to a higher mutation rate, you are correct. But that was not implied by the comment you're quoting.

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