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> which is why this gene has not been bred out of the population over time.

Is that why? Or is it just the people with it aren’t sick enough to die before procreating?




It's a recessive/heterozygous thing. If you get the gene from neither parent, you're vulnerable to malaria. If you get the gene from either parent, you're immune to malaria and don't get sickle cell. If you get the gene from both parents, you get sickle cell. A hypothetical future person who's going to be born in an area with a lot of malaria would really want exactly one parent with sickle cell and one parent lacking the gene completely to guarantee the best personal outcome, or they'd want exactly one heterozygous parent (for a 50% chance of being immune to malaria with no downside), or they might settle for the gamble of two heterozygous parents (50% chance of immunity, 25% chance of sickle cell).


(You already know this but for the general audience)

The train doesn't make you immune to malaria but it does increase resistance after infection.


This sounds like Tay Sachs for Africans. Read that carriers of Tay Sachs might have defended them against tuberculosis, and they're also looking at gene therapy for it.

Prevention is the preferred method of passing this trait on however.


Can they do sperm (or egg) selection to change those odds for IVF?


In fact yes! https://punchng.com/value-of-ivf-in-elimination-of-sickle-ce...

But practically, it'd be a huge challenge. Nigeria's one of the main victims of malaria and, not by coincidence, one of the main victims of sickle cell. There are IVF clinics in Nigeria, but they're very expensive even before you consider sickle cell testing. It likely wouldn't scale to all of the births per day, and something like a quarter of the country would need it.

But it's not IMPOSSIBLE. You'd need to do maybe 75 or so per day to cover the 25% or so of the country that have the gene and would need it. Hard and expensive and impractical, but perhaps possible?


Or add the DNA tests to dating apps.


Without access to modern hospital treatments it is fairly normal to die very young from sickle cell disease - it causes 100k+ deaths a year.

An in-law of an ex has it, and regularly spends days in hospital during crises. Without access to a high quality hospital he'd have been dead a long time ago.

The average life expectancy for someone with sickle-cell disease in developed countries is 40-60 years, and serious crises tend to start from childhood.

That said, it's recessive, and so it's likely the reverse of what you think: It's not primarily the people with full-blown disease who contributes most to the long term survival of the trait, but that the trait alone confers fairly significant advantage in regions where Malaria is huge killer mostly without causing health problems. So across the combined set of carriers and those with the full disease, the life expectancy in Malaria stricken areas tends to be higher.

Pattern of change of the prevalence of the trait correlating with changes in prevalence of Malaria has been observed many places. E.g. the prevalence among US black people is significantly lower and dropping than in the areas their ancestors came from.


I think this is right, but just to spell out the recessive gene implications for readers, here's the Punnnett square[1] :

      R  | r
    +----+----+
  R | RR | Rr |
  --+---------+
  r | Rr | rr |
    +---------+
The people with sickle cell disease are "rr" — that's 1/4 the population.

The people who have some malaria resistance are all of the ones with "r". In particular, the "Rr" folks have the resistance, but not the anemia.

So basically, this gene screws over 1/4 of the population and benefits 1/2. In areas with lots of malaria, this tradeoff is worthwhile, evolutionarily speaking.

One of those harsh cases where evolution (if we personify it) does not care about individuals — only the species.

[1] https://en.wikipedia.org/wiki/Punnett_square


Worth noting punnet squares are kind of bunk: https://youtu.be/zpIqQ0pGs1E?si=SDRQP-PW2u_6Jq3d

Although sickle cell does seem to be one of the rare cases where they work out.


Punnet squares are as "bunk" as Ohm's Law.


Malaria often kills people before they reach the age of being able to procreate.


No the important part is that the mutation is recessive, but being heterozygous for it is enough to confer malaria resistance.


In regions where malaria was endemic, this mutation was selected for, as malaria kills children.


Africa wasn't colonized by Europe until vaccines and treatments were invented because of malaria and other tropical diseases. Quinine was one of the last ingredients needed to conquer Africa.




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