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February 28, 2005

Taste & behavior genetics

As regular blog readers know I am something of a chili pepper addict. I have previously mentioned the genetics of taste, particularly in relation to phenylthiocarbamide (PTC) sensitivity, which correlates strongly with a variety of responses to bitter, salt and sweet sensations (you probably tasted a piece of paper in high school biology which had some PTC on it, the reaction for tasters was reflexive revulsion, while non-tasters didn't understand what the fuss was about). The key is some people tend to be hypersensitive to bitter tastes, others less so, and a minority of people relatively insensitive.

My interest in this topic was prompted by the fact that I am both a PTC non-taster and rather adept at consuming large quantities of chili. Capsaicin, the active ingredient in pepper that gives it the "fire" taste, selectively effects mammals but not birds. In an evolutionary context this promoted wider dispersal of the pepper seeds. Therefore it is surprising that some humans seek out the taste of capsaicin (though it does have anti-bacterial properties). In any case, while in college I noted there was some evidence that there was a relationship between PTC insensitivity and capsaicin insensitivity. Additionally, I noted that South Asia had a high number of PTC non-tasters, so I inferred that South Asia had a genotypic predisposition to adding chili pepper to its suite of condiments (it is a New World import despite its modern ubiquity in much of Asia). Digging around did not show any startling interpopulational patterns, though there is a lot more to PTC than just capsaicin.

So, I was interested when I noted two articles related to PTC (and its cousin PROP) had been published. The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception connects the dots between the phenotype and the genotype. Crucial, but not too sexy. On the other hand, Genetic and environmental determinants of bitter perception and sweet preferences is one of those papers where the abstract gives much of the game away. Some conclusions:

  1. Taste sensitivity is likely mediated by environment. Children showed a stronger genotype-taste discernment correlation than adults. This should not surprise anyone, we all know we can "acquire" tastes. My spice tolerance has ratcheted up greatly since childhood, while many people get over the bitterness of beer.
  2. Children with genotypes that predisposed them to taste sensitivity were the most likely to prefer sweet drinks. This pattern was not found in adults.
  3. Race was a significant variable as black children reported far greater preference for high levels of sugar in their cereal. This indicates a strong cultural factor, but, I recall that much of West Africa had a low frequency of the insensitivity that is correlated with lack of craving for sugar, so you might be seeing a gene-environment correlation here as differences in genotypic distribution have had a multiplier effect in shaping cultural culinary habits (though spice is common in black American and apparently much of West African cuisine, so we shouldn't make much of the high number of PTC/PROP tasters in West Africa).
  4. When mother and child differed in their taste sensitivity it affected their interaction. In particular, taste insensitive mothers perceived taste sensitive children (they are focusing on one locus and two allelic variants in a standard homozygote-heterozygote model with additive effects) as being more "emotional" than taste insensitive children. This is important, because it is likely a hint that small, seemingly trivial, genotypic differences can leave a wider footprint on the extended phenotype of a child and parent.

Previously I focused on intergroup differences on this locus (a table of frequencies if you are curious), but I am now curious as to further study in more convential behavior genetic modes.

Related: Natural Selection and Molecular Evolution in PTC, a Bitter-Taste Receptor Gene, PTC taste, balancing selection?, PTC, part II and Genetics of taste.

Posted by razib at 12:02 AM