Gene Expression

Genes & culture & milk

Gene-culture coevolution is a topic of interest for me. Consider adult milk digestion. It’s weird, and seems like a new adaptation. The lactase gene has been under such strong selection that it is often used (or the region around it) as a control to test whether new methods for detecting selection actually work where we think they should work.

Here are a few maps I reworked from this paper:
i-03da46de0befe4dcbe1f7e2a78b4a146-lactosemap.jpgThe area where cattle genes which produce milk are diverse is relevant because that is the region where milk producing cattle have likely been resident the longest. The logic is similar to why Africans are assumed to be the source population for other humans: their genetic material exhibits the extant variation of an ancient population which has been resident for a long enough period to build up mutations. Note the close correspondence!

Now, recently I stumbled onto to this old paper about differences in lactose tolerance in north and south Indians. The range given seems to be about 75% lactose tolerance in northern India vs. 35% in southern India. What’s going on here? Some make a phylogenetic argument: lactose tolerance is a signature of Aryan immigration. The problem with this argument is that South Asians are fundamentally closer to each other than they are to outside populations, and the vast majority of ancestry seems to derive from around the Ice Age or before. Though some exogenous genetic material can be found in northern Indians which is derived from populations to the north & west, the extent does not predict the level of lactose tolerance that we see. If one assumes that Indians were not lactose tolerant originally and it was introduced than south Indians would exhibit at least 35% admixture, while north Indians would be 75% exogenous (assuming really simple genetic models obviously). This isn’t warranted by any of the other data on other loci. Additionally, the recent genomic work on the lactase genes suggests the unity of the origin of Eurasian lactase persistence. In other words, the genetic strategy in Eurasia, the T allele on LCT, appeared once, and spread (in Africa there seem to be other strategies). But if other genes don’t support massive admixture between various Eurasian populations…what happened? Gene flow, and a selective sweep of a favored allele! India has many cows, and dairy is part of the diet, but this is most prevalent in the northwest where lactose tolerance has the highest penetration. When the T allele entered India its fitness was very high in the north, and less so in the south. So it rose to high frequency driven by positive selection just as it has in many other cultures.

Comments

  1. #1 fred
    November 23, 2006

    Great map. Is there anything like this for goats? Regions that are reliant on goats’ milk, do they show similar patterns, is goats’ milk general across populations?

  2. #2 dougjnn
    November 23, 2006

    Gene flow and selective sweep of a favored allele looks like a very powerful and fairly fast acting factor.

    Nice map though what happened to Britain? I suspect the Frogs.

  3. #3 dougjnn
    November 23, 2006

    Fred said–
    Great map. Is there anything like this for goats?

    I suspect that lactose tolerance or not applies to goats about as well as to cattle, just as it does to mother’s milk.

  4. #4 MJ Memphis
    November 24, 2006

    http://upload.wikimedia.org/wikipedia/en/7/7e/LacIntol-World.png

    Here is a bigger (although less detailed) world map for lactose intolerance. Although I notice that Thailand is listed as having almost 100% lactose intolerance, which is definitely not the case, so take it with a grain of salt.

  5. #5 Ken Hirsch
    November 24, 2006

    Well now we know why Vikings wore helmets with horns. It was part of the “Got Milk?” dairy council promotion!

  6. #6 dougjnn
    November 25, 2006

    I’m sorry but that Wikipedia map on lactose intolerance/tolerance strikes me as patent junk. On the basis of background knowledge and common sense.

    Scandanavia and Switzerland are the only areas of Europe with more than middling amounts of lactose tolerance, while the US is way up there? Those cheese eating frogs are in the undistinguished middle together with the Italians and the Danes? Ireland and Britain are in the middle group but Australia (composed overwhelmingly of those ethnicities) is like the US solidly in the lactose tolerant group. COME ON.

    Conflicts with common sense. Sure it takes more than that to disprove but the smell test is massively failed.

    One of the more suspect Wikipedia entries I’ve ever seen in fact.

  7. #7 Agnostic
    November 25, 2006

    Re: Switzerland, etc. — hard cheeses (like Swiss) don’t contain much lactose, and the lactose intolerant can usually eat them w/o trouble. It’s really untreated milk that’s difficult for the intolerant.

  8. #8 dougjnn
    November 25, 2006

    Ah, ok. Now that Wikipedia map on latcse tolerance/intolerance makes more sense.

    I was reading the grey areas which includes almost all of Europe as middle levels of lactose intolerance by scanning the color code bar and noting it’s roughtly colorless middle. But upon just now taking another look and blowing the bar up bigger to be able read the labels better there’s also a separate distinctly grew entry at the very bottom labeled “no data”.

    So instead of being wildly improbable that chart is just strangely clueless about the high levels of lactose tolerance in Europe and a number of other places.