I've heard this in classes I took since I was an undergraduate (i.e. decades). However, I've never looked into the primary research literature about this. In other words, I've never checked for myself whether this is true! Here is a summary of what I've heard.
Humans are diploid, which means we have two (di-) copies of each gene. A benefit of diploidy is that if one copy of a gene sustains a DNA sequence change (a mutation) that prevents it from producing a normal protein, we have a second copy of the same gene. In many cases, perhaps, that remaining copy will suffice. If a person has one normal gene and one mutant copy, and if that person is OK, then that person is called a "carrier." The carrier seems normal, but they have a mutation that could be passed on to their children - and their children could manifest the effects of that mutation.
One genetic disease that follows this pattern is sickle cell anemia. In red blood cells, the protein hemoglobin (Hb) binds oxygen, and then the red blood cells deliver that oxygen to our body's tissues. One mutation in Hb causes that protein to fold into a non-normal shape (it looks like a sickle under the microscope), so it called the HbS mutation. When a human has one normal (denoted as +) and one HbS version of hemoglobin, the normal hemoglobin works fine, and the fact that the HbS hemoglobin does not function properly has no detectable effect on oxygen delivery. This is a carrier individual. They are referred to as geneticists as being +/HbS (one + or normal version, and one HbS mutant version, with the forward slash / being used to separate the two notations).
So, there are other two possible genotypes: +/+ and HbS/HbS: a person can have two normal versions of hemoglobin (which is the common situation), or they could have both mutant versions of hemoglobin. These latter individuals are the ones who develop a genetic disease called sickle cell anemia. The lack of normal hemoglobin causes the misshapen red blood cells to clump together in the bloodstream and cause excruciating pain.
In the 1900s, scientists detected a conundrum: if sickle cell anemia is a genetic disease, then why did so many people have it? Normally, mutations that elicit a disease are removed from the population of humans through natural selection. In other words, if somebody had sickle cell anemia, we might expect either that they would die before being able to reproduce and pass the mutation on to their children or at least that an individual who had sickle cell anemia would choose not to have children, since their children would definitely be carriers (if not afflicted).
It appears that things are slightly more complicated. Yes, +/+ individuals are healthy, and HbS/HbS individuals have sickle cell anemia. But the story of the carriers is fascinating. Not only do carriers not manifest sickle cell anemia, but also they seem to be resistant to malaria. If true, then this would explain the prevalence of sickle-cell anemia mainly in regions of the world where malaria is also prevalent: a cost-benefit relationship. Despite carriers being beneficial in environments where malaria is common, they also occasionally produce children with sickle-cell anemia, but they will produce more carriers than sickle-cell children, so the mutation is maintained.Student Responses
Not surprisingly, the genetic basis of malaria resistance has been a widely-studied topic, so there are lots of research papers to read (and the students found many). The oldest paper (Allison 1954) raises the same question as above: why is the HbS mutation not being eliminated from the population by natural selection? Allison noted several previous studies that had identified a correlation between the prevalence of sickle cell anemia and malaria, but apparently the potential for a causal relationship had not been tested. In his study, humans with and without sickle cells were purposefully exposed to malaria, and Allison observed that malaria almost never occurred in individuals with sickle cell anemia, where it was common in those who did not. The paper concludes with the extrapolation that carriers might benefit from protection against malaria.
Subsequent studies have further tested this hypothesis. For example, Ferreira et al. (2011) found the same effect in mice: that the HbS mutation confers malaria tolerance.
Finally, many students identified review articles that debate the mechanisms by which HbS confers malaria resistance - the answer is still unclear (but see, for example, the recent study by Archer et al.).
Student Decision: Fact or Fiction?
Fact
Literature Cited
- Allison (1954) "Protection afforded by sickle-cell trait against subtertial malarial infection." British Med J 1(4857):290-294.
- Archer et al. (2018) "Resistance to Plasmodium falciparum in sickle cell trait erythrocytes is driven by oxygen-dependent growth inhibition" PNAS 115(28):7350-7355.
- Ferreira et al. (2011) "Sickle Hemoglobin Confers Tolerance to Plasmodium Infection." Cell 145:398-409.
