If all humans are genetically similar...

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If all humans are genetically similar...

#1  Postby Symbiont » Dec 04, 2022 2:07 am

If all humans are genetically similar; why is inbreeding bad?

or

Since we're 99.9% genetically similar as is assumed. Then inbreeding shouldn't be an issue but it obviously is and since inbreeding is an issue. Then we humans aren't 99.9% genetically similar.

Am I missing something?
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Re: If all humans are genetically similar...

#2  Postby Evolving » Dec 04, 2022 6:54 am

That's a good question.

I mean, clearly we are "99.9% genetically similar".

I daresay it's very straightforward to answer, I look forward to someone doing so.

Many years ago I asked on this site how it was possible for the number of chromosomes to change through evolution - it's a discrete number, it doesn't change in infinitesimal steps - how do the mutants then breed? It turned out that the answer wasn't particularly straightforward, but there was one.
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Re: If all humans are genetically similar...

#3  Postby Fenrir » Dec 04, 2022 7:28 am

Inbreeding has little to do with overall variation within a species (and "99.9% similar!!1!11" doesn't really say much, see if you can work out why). Inbreeding is due to mating of very closely related individuals and leads to homozygosity of recessive traits. Ultimately such traits are removed from the population by negative selection (also called purifying selection).

Humans are bright enough to recognise many homozygous recessive traits and take steps to ensure purifying selection does not occur, because we are an empathetic species and turned off by nasty debilitating conditions leading to suffering and early death, like Sickle Cell Anemia, Tay Sachs disease, voting republican etc. etc.

Generally these steps can be summed up by "don't shag your siblings or your parents". Even the authors of ancient mythology managed to figure that one out.
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Re: If all humans are genetically similar...

#4  Postby Spearthrower » Dec 04, 2022 8:40 am

Symbiont wrote:If all humans are genetically similar; why is inbreeding bad?


Genes with very harmful traits aren't expressed when they are recessive, only when there's 2 copies of them.

In some populations, these traits can survive perfectly well while recessive as they either don't harm the individual, or maybe even offer some other benefit.

So if a particular nasty gene is fairly common in a population, and if individuals in that population continually breed among themselves, the chances of these harmful traits being expressed increases.


Symbiont wrote:
Since we're 99.9% genetically similar as is assumed. Then inbreeding shouldn't be an issue but it obviously is and since inbreeding is an issue. Then we humans aren't 99.9% genetically similar.

Am I missing something?


Aside from the point above, this isn't very clear reasoning.

It's equivalent to saying that 99.5% of the time you drive your car, you don't have an accident, and concluding that therefore vehicle accidents can't occur. It's not the 99.5% that's the problem, it's the 0.5% you have to account for.

There are 3 billion base pairs of DNA in the human genome.

99% of 3 billion is 2,985,000,000 that still leaves 15,000,000 which differ. On the occasion when some of those remainder happen to include a dominant and recessive copy of a deeply harmful trait, then bad results occur.
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Re: If all humans are genetically similar...

#5  Postby Evolving » Dec 04, 2022 9:06 am

Thank you, that does make a lot of sense.

Like (I presume) the OP, I was focussing on the idea that each of us has almost the same DNA as all other members of our species (what is wrong, by the way, with saying that we are 99% genetically similar? we frequently see statements along the lines that we humans share x% of our DNA with chimps, y% with lettuce, etc). And that led me to the faulty conclusion that the problem with inbreeding was limited to the 0.something % that is not identical.

Evidently that's wrong: a recessive gene can be anywhere along the genome; to what percentage on average we share our DNA is irrelevant.
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Re: If all humans are genetically similar...

#6  Postby Spearthrower » Dec 04, 2022 9:47 am

what is wrong, by the way, with saying that we are 99% genetically similar? we frequently see statements along the lines that we humans share x% of our DNA with chimps, y% with lettuce, etc


There's nothing so much wrong with it as it being sort of a confounding question when, as you note, we also share a massive suite of DNA with other species, largely in accordance with how far removed we are from a common ancestor (although it's not 1:1).

The bits that are specific to humans, or in which variation impacts humans in particular, would be a better place to start from when considering the question, because that's where there's variation among humans, which then makes sense of a question that asks that conditionally!

I shouldn't have toked before writing that sentence! :doh:


a recessive gene can be anywhere along the genome


Correct, but there's also the element of purifying selection Fenrir raised; there's no real general statement that could be made here, but you can imagine that some of the most ancient shared genes have undergone many, many rounds through many species, and their effect was so harmful as to have effectively made themselves extinct in the population and all daughter populations. Mutations can still produce this harmful allele, but the chances of it finding itself in a body with another copy is extremely low.

Other genes, often more recent in evolutionary terms, or those with more complicated effects such as conferring some benefit, may have more copies in the population to encounter and thus are more likely to persist in the population and... feedback loop. This is magnified by geographical proximity when individuals continually breed with neighboring relations who are more likely to have that gene; it ratchets up quickly under certain conditions, and inbreeding can quickly become endemic to small populations as there's literally no one to breed with absent those genes.

All species have to manage this in some way, and there's quite a lot of interesting evidence about our earliest modern human ancestors practicing female exogamy, or marrying daughters out to neighboring tribes.
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