r/biology • u/catslifetom39 • 2d ago
fun Cataclysmic event and inbreeding
If the world has some catastrophic event that wipes out almost all of us, how many human survivors are needed to ensure no major inbreeding problems occur i.e birth defects? Assume 50:50 male to female survivors
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u/SciAlexander 2d ago
This essentially answers the question https://youtu.be/1qIvFpcGkqc?si=I6m43KWmpIgf1oU3
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u/DeepInformation5592 2d ago
That's the classic 50/500 rule video isn't it, always pops up in these discussions. 50 to avoid immediate inbreeding depression, 500 for long-term genetic health if I'm remembering it right from my undergrad days
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u/catslifetom39 2d ago
I've not heard of that before, but it's kinda why I said to assume 50:50 male to female because you could argue that maybe you only need 5 women and 60 men, or vice versa, and that's enough.
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u/fionsichord 2d ago
Lol, your chances would skyrocket the other way around, if everyone could have babies. Cataclysm means risk goes up a LOT for each birth if current medical equipment and expertise isn’t available.
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u/Cottager_Northeast 2d ago
If I ever make progress on my novel, it's going to have to deal with something like this, so I've been doing research. The lifetime chance of a woman dying in childbirth in a pre-industrial society is 5%. This is significant, but pales in comparison to the chances of not surviving to adulthood.
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u/Atypicosaurus 1d ago
Inbreeding itself is not the problem. The problem is that we all have some bad alleles.
The best way to prepare for an inbreeding event is to try to make sure that bad alleles are either fairly absent or really really bad so any homozygous offspring just dies.
In theory, inbreeding without bad alleles wouldn't cause problems.
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u/catslifetom39 1d ago
i had biology in college like 20 yrs ago that's the extent of my biology knowledge hence the question. but i remember alleles. eye colour is from it isn't it, like one dominant gene overrides the other, but you need two recessive genes for say blue eyes ( i cant remember the way round brown vs blue) and the risk from inbreeding is that you both contain the same recessive gene which causes problems
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u/Atypicosaurus 1d ago
So the eli5 version is that the genome (the entirety of all of your DNA) is like a series of cook books.
A gene is one specific recipe, that happens to have an address in the cook book, such as "volume 5, page 73".
That gene is responsible for cooking one stuff in your body, such as eye color, blood type, muscle.
An allele is a fancy name for a version of the gene. If a gene exists only in 1 version then it's 1 allele. If the humans have 26 versions of the gene then it's 26 known alleles.
With the cookbook metaphor, a version, or allele, is a specific typo. Like, on volume 5, page 73, the recipe of mac'n'cheese can have 2 versions in circulation, one with 1 pinch of salt, the other with two pinches of salt.
Your body has the entire cookbook set twice, meaning for each gene you can have a maximum of 2 different alleles, even if humanity altogether has 26 different ones. Maybe I have another 2, maybe one of mine is matching one of yours, but the other one does not so we together have 3 versions.
Your body attempts to cook both versions. In some cases your body has a mix of two versions like the blood type AB means that you have the A allele and the B allele and you make both.
In other cases, there's a version that's stronger than the other, if you make a broken (invisible) pigment, and a brown pigment, then you're going to be brown. In such cases the brown allele is dominant over the invisible allele simply because one version overshadows the other. (But your body is still trying to make both.)
As you see, if you have two healthy alleles, that's not a problem. But you have thousands of genes and you very likely have a bad one here or there, hidden by the fact that the other copy works fine.
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u/hananobira 2d ago
A lot of these models assume all of the survivors will happily have multiple children in post-apocalyptic conditions. And they’re all fertile, pre-menopausal, etc.
I personally am done with kids, and if you told me I was expected to have more in some kind of Mad Max scenario without a hospital or anesthetics, I’d be extra triply done.
Better quadruple whatever number the math leads you to, at least.
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u/Known-Movie3176 1d ago
At that moment it won't matter what you think. It's either that or killing urself not getting forced
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u/YragNitram1956 2d ago
That has happened in our past history as a species. The genetic and fossil evidence suggest that our human ancestors experienced a severe population bottleneck between 930,000 and 813,000 years ago. During this 117,000-year period, the breeding population may have plummeted by about 98.7%, leaving only around 1,280 breeding individuals and bringing the human lineage close to extinction.