Wednesday, August 07, 2013

Musings on Lactose Persistence

An interesting map of lactase persistence from Nature:

Differences in Gene Expression & Hybridization
An interesting things from the article:
Most people who retain the ability to digest milk can trace their ancestry to Europe, where the trait seems to be linked to a single nucleotide in which the DNA base cytosine changed to thymine in a genomic region not far from the lactase gene. There are other pockets of lactase persistence in West Africa (see Nature 444994996; 2006), the Middle East and south Asia that seem to be linked to separate mutations3

Since lactase persistence in West Africa, the Middle East and South Asia all seem to be linked to separate mutations, I wonder:

  1. if the different mutations for lactase persistence are differently efficient, and
  2. if people who have parents from different lactase hotspot regions have significantly different lactase production capability.
Since the mutations of lactase persistence occurred independently in these different populations, it means that it is possible that each population might have a different efficiency of producing lactase. (It's not necessarily true, but it is possible.) For example, if Northern European populations could digest x milliliters of cow milk in t seconds, would West African populations be able to digest the same quantity in the same amount of time? Would this hold for all quantities? For commonly drunk types of milk (e.g., cow milk, goat milk, sheep milk, mare's milk, camel milk, etc.)? And for all hotspot populations? These would be interesting things to check out (especially if you were a cereal company who wants to open new markets...)

With regard to the question of whether genetic hybrids would have better capacity of producing lactase, it is important to determine whether the trait is dominant or recessive. In a comparative study between the Northern European and the Middle Eastern mutations for lactase persistence, the authors assume that the genetics of both mutations are dominant traits, which would mean that only one gene is necessary for the trait to be expressed. If a person had both the Northern European mutation and the Middle Eastern mutation, therefore, it is likely that they could well express both lactase persistence mutations. (The assumption of dominance is also more likely, considering that upwards of 90% of the populations in the hotspots can digest lactase, which strongly implies that it's a dominant trait, since a recessive trait is unlikely to emerge at such a high rate, unless there were some strong selective pressures for that trait.)

The Mongol Question
In another note, the article is mum about Mongolia. Indeed, the shading of lactase persistence is at a pretty low resolution (as it is for much of Asia), but it estimates it at around ~45%. This strikes me as a little odd, since much of the population does consume mare's milk, and some regions also consume yak milk and others consume camel milk (i.e., in those regions where they herd yak and camels, respectively). From these milks, different products are made, which serve as a major food source throughout the year. Now, maybe ~55% of the Mongolian population is lactose intolerant, and just go around with the runs and bad gas all the time, but I couldn't find any information about Mongolia. Instead, when I looked for lactose tolerance data for Mongolia, I kept pulling up a 1984 study of ethnic populations in China, including Inner Mongolia. One note, though: Inner Mongolia has a very different demography than the nation of Mongolia, and it's not clear from the reviews of the study that the authors tested ethnic Mongols (who are a minority in Inner Mongolia), Han Chinese (who are the dominant majority), or other minorities (e.g., Manchus, Hui, Daur), or even if they determined the level of hybridization of the subjects (since determination of ethnicity is a social construction - in which a person who is half-Han could still be considered to be Mongol and other ethnic minorities are also considered to be Mongol - whereas lactase persistence is based on biology). I'm going to guess - based on only anecdotal evidence and inference - that the degree of lactose persistence in Mongolia is higher than that of Inner Mongolia and that it could well be higher than the ~45% indicated on the map.

Even though the resolution of the data is pretty sparse in Asia (apart from South Asia), it's interesting to note that if we assume that the darker band of lactose persistence in northern and eastern Asia are due to the Mongols (which I think is justifiable, based on the wealth of evidence that Mongols do consume significant amounts of milk products), then the dark spot of lactose persistence in Japan's main island is additionally interesting. If true, it would appear to bolster some minor lines of genetic evidence that link Japanese populations to Mongolia and potentially explain why lactose persistence is a far more common trait in Japan than it is in the nearby Korean peninsula.

In any event, it would be interesting (and somewhat fulfilling) to see more data from northern Asia (especially from the herding ethnic groups found in that vast - and under-sampled - area).

Of course, I'm not a geneticist, but when I see maps like these, they make me wonder about the interesting implications of genetics.

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