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Obesity Rate Levels Off

After several decades of constant growth in the past decade obesity did not grow any more among children [1] and adults [2]. Some authors would claim banning vending machines from schools and other such restrictive measures responsible [3].

I think that obesity is a polygenetic disorder, the alleles that determine the phenotype have a certain prevalence in our population. For centuries these alleles have been being kept in check by insufficient food supply. Now with a population's growing wealth there was no more external obesity control and slowly but unremittingly the phenotype of a population developed according its genotype. I'm convinced the prevalence of obesity alleles is not 100%, so we would never by any measures end up with a population of 100% obese people. On the contrary, I guess the prevalence of obesity genes is about 25% that is reached about now, so I doubt that banning vending machines from school had any measurable effect.


1. Cynthia L Ogden u. a., „Prevalence of Obesity and Trends in Body Mass Index Among US Children and Adolescents, 1999-2010“, JAMA: The Journal of the American Medical Association (Januar 17, 2012), http://www.ncbi.nlm.nih.gov/pubmed/22253364.
2. Katherine M Flegal u. a., „Prevalence of Obesity and Trends in the Distribution of Body Mass Index Among US Adults, 1999-2010“, JAMA: The Journal of the American Medical Association (Januar 20, 2012), http://www.ncbi.nlm.nih.gov/pubmed/22253363.
3. Sarah Kliff, „Why are obesity rates leveling off?“, The Washington Post - Blogs, Januar 18, 2012, http://www.washingtonpost.com/blogs/ezra-klein/post/why-are-obesity-rates-leveling-off/2012/01/17/gIQAT0GF6P_blog.html.

Tags: Genetics

Selection of Polygenetic Traits

Polygenetic selection describes the changes in a gene pool in response to adaptation of a polygenetic trait.


Let us assume we consider a polygenetic trait that is determined by several independent loci.

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The alleles that favor the trait are marked red and numbered CVL1-8.

Let us further assume that there is a linear relationship between the number of CLV allele per individual and the probability to phenotypically expose the trait.

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The green line in this graph depicts such a linear relationship. I'm fully aware that this is an oversimplification. The relationship might be non-linear convex as the yellow line or concave as the red one. Next, the relationship always is a statistical one. That is our line depicting relationship is only the correlation function of the cloud of single observations (black dots). Finally, I'm fully aware that this models neglects probability functions that exist for all alleles, and have to be described by a set of conditional probabilities.

All this can be neglected for the sake of comprehension only. The principle of polgenetic selection would hold even if we consider a quite more complex model. (The pioneers of polygenetic inheritance, including Waddington, considered a model with even a simple threshold function. (Unfortunately, many people still believe that all genetic inheritance works like Mendelian traits, which hinders them to understand evolutionary principles.)


The selection of polygenetic traits leads to an accumulation of alleles that favor the trait that is positively selected.

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In this picture two populations are shown. In the population on the left the the alleles that code the trait are scattered scarcely while after the selection process, the population on the right, the alleles are much more common. Each individual is shown as a single chromosome. It really doesn't matter if these alleles summon on a single chromosome by crossing over or on different chromosomes of the same individual.

Experimental Example

The Waddinton experiment is an example of slecetion of a polygenetic trait, the crossveinlessness. Waddington enhanced the process of selection by arteficially increasing the expression of the phenotype even in individuals with a lower number of specific trait alleles.

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Graphical representation of his radiation experiment would be an upward shift of the curve. Again, it doesn't matter whether this shift in probability of expression is caused by a linear shift (blue line) or by the curve becoming more convex (cyan line).


(c) Mato Nagel, Weißwasser 2004-2023, Disclaimer