Genetic determinism: mosquito bite edition
Q: Did you see that being bitten by mosquitoes is genetic?
A: What? Being outside without mosquito repellent, especially in the evening, is genetic now?
Q: No.
A: Living in places with lots little pools of standing water in the summer is genetic?
Q: No
A: Having wire mesh screens on your windows is genetic?
Q: Ok, yes, very droll. No, “Scientists have found that the chance of being bitten by a mosquito is written in the genes and some people are just more likely to be attacked no matter how much insect repellent they slap on.” What repellent did they use? DEET or one of those lemon things?
A: You mean this paper in PLoS One. They didn’t use any repellent.
Q: So they don’t really know that the usual repellents don’t work for some people because of genetics?
A: No. They didn’t look at that at all.
Q: Should I pretend to be shocked?
A: Don’t bother for now.
Q: Ok, who got bitten by the mosquitoes? You’re not going to tell me it was mice again, are you?
A: No, no mice, but also no bites. The researchers took smell samples from volunteers’ hands, and measured which samples the mosquitoes flew towards?
Q: How did they choose the people?
A: The comparisons were all within sets of female twins.
Q: Not really a representative population sample, was it?
A: It’s a reasonable approach for testing if there’s a genetic component to something: identical twins should be more similar than non-identical twins.
Q: And if it was completely genetic, identical twins would be identical, right?
A: Yes.
Q: So when they say “the mosquitoes would bite none, or both of the identical twins, but the results were mixed for the non-identical twins” that means it was completely genetic?
A: No, the implication doesn’t work backwards that way. And also that’s a pretty serious exaggeration of what they found.
Q: But at least they did find a genetic component? Some people make a natural repellent?
A: There’s pretty good evidence of a genetic component, even a fairly big one. The article makes it clear that they don’t know whether some people make a repellent or whether other people make an attractant: “It is not known whether the differences between MZ and DZ twins is due to the presence or absence of attractive or repellent chemicals,”
Q: That seems pretty unambiguous, but it isn’t what the newspaper says.
A: No, it isn’t.
Q: Should I pretend to be shocked now?
A: I’d wait and get it over with all at once.
Q: The newspaper has a link to a related story. Should we read that?
A: Sure. Why not?
Q: It says “most of this research uses only one mosquito species. Switch to another species and the results are likely to be different.” Huh. I didn’t know that. Which species did the twin research use?
A: Aedes aegypti, a tropical mosquito (originally from Africa) that spreads yellow fever and dengue.
Q: Is Aedes aegypti common in New Zealand?
A: No, it could live in Northland but the biosecurity folks have stopped it invading so far.
Q: How about in the UK, where the news story came from initially?
A: No, the UK is too cold.
Q: So it’s not really relevant to mosquito bites for their readers?
A: It’s still important as a global health issue, but no, not all that relevant from a mundane viewpoint of actual day-to-day utility.
Q: Ok, is now the time to pretend to be shocked?
A: If it makes you feel better, sure.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »