Misunderstanding genetic heritability
From the Herald, under the headline “Is this why we’re all getting fat?”
According to the UN’s World Health Organisation, obesity nearly doubled worldwide from 1980 to 2008.
More than 2.8 million adults die each year as a result of being overweight or obese, it says. A full 42 million children under the age of five are considered to be obese.
Diet and a sedentary lifestyle have long been fingered as causes of obesity, but in recent years, advances in gene sequencing have turned attention to inheritance.
Previous studies have variously estimated genes as being to blame for between 40 and 70 per cent of the problem.
Every sentence here is true, but the impression is completely wrong.
The 40-70% genetic contribution to weight is comparing different individuals in basically the same environment. The ‘obesity epidemic’ is comparing whole populations over time. One thing we know can’t possibly explain the recent increases in obesity is genetics: there hasn’t been time for the genes of these populations to change.
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 »
Thank you very bringing this up. I am on the research team at the Behavioral Health and Wellness Program at UC Denver, Anschutz Medical Campus. As a semi-regular part of my job, I give a training to social workers, nurses and other professionals on healthy eating and weight management. I have to explain this in some detail in nearly every training. It seems like such an obvious thing if you have only a remedial understanding of genetics and heredity (which is *all* I have) and yet it really, really isn’t. So frustrating to see these true/not-true stories out there gumming up the works for the frontline workers.
PS: Very happy I found your blog, btw. Great work here.
9 years ago
What’s the timeframe for epigenetic changes? Based on studies such as this http://www.fasebj.org/content/27/1/350 can we suggest that “genetic” changes can happen within a generation?
9 years ago
The differences described in the story aren’t epigenetic, they are genetic.
There could be epigenetic changes, but for this purpose that’s a subset of environmental not of genetic.
9 years ago
Heritablity is an attribute of a population in a particular environment, and tells you little about an individual.
Heritability estimates are most useful if you want to breed fatter chickens and cows which give more milk without changing the environment. Change the environment and the estimated population heritability value changes. Thus heritability estimates are not useful in the situation with humans where you want to understand what to change (in the environment rather than by selective breeding!) to achieve some outcome. The best example of this is the heritability of PKU which dropped to near zero after an environmental (diet) intervention even though it has a known genetic cause.
There are independent estimates of between group heritability and within group heritability, and the two are unrelated (or independent) in the same way that ecological correlations are different from within group correlations. It’s possible for the differences between populations to be predominantly environmental and the differences within populations to be predominately genetic, and vice versa. No need to invoke genetic changes or worry about timeframes.
My PhD in 1983 attempted to calculate within and between group heritability for 27 anthropometric measurements in 6 populations in the Solomon Islands (it’s in the U of A library entitled Quantitative Genetics of Anthropometric Variation in the Solomon Islands). Making comparisons across/between populations using heritability estimates from within a population is not a sound way to proceed.
The utility of heritability is still misinterpreted despite the issues being worked through in the mid 1970s. *sigh*
9 years ago