The heritability of a trait within a population is the proportion of observable differences in a trait between individuals within a population that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics (in their “phenotypes”)…Heritability thus analyzes the relative contributions of differences in genetic and non-genetic factors to the total phenotypic variance in a population. For instance, some humans in a population are taller than others; heritability attempts to identify how much genetics are playing a role in part of the population being extra tall.
Over at Haldane’s Sieve Dr. Joseph Pickrell has a commentary up on a preprint on explaining the ‘missing heritability’ using yeast genetics. All good reading. I long ago gave up on the idea that the idea of ‘heritability’ would ever be widely internalized among the educated public in any precise sense. But we muddle on. The next decade is going to be big for the genomics of complex traits. Or so people keep telling me!
But this gives me the excuse to point to a commentary which you really should read again and again. It is A commentary on ‘common SNPs explain a large proportion of the heritability for human height’ by Yang et al. (2010).:
The Pith: A great deal of important medical genetic differences between people may be due to the nature of interactions of genetic variants.
If you’ve been reading this blog for a while you know that there is a question in genomics right now as to “missing heritability.” The issue is basically that there are traits where patterns of inheritance within the population strongly imply that most of the variation is due to genes, but attempts to ascertain which specific genetic variants are responsible for this variation have failed to yield much. For example, with height you have a trait which is ~80-90 percent heritable in Western populations, which means that the substantial majority of the population wide variation is attributable to genes. But geneticists feel very lucky if they detect a variant which can account for 1 percent of the variance.
One simple explanation, which gains some genomic support, is that variation on these traits is due to innumerable variants widely distributed across the genome. Therefore, a variant of one percent effect may be a rather large one. There are also those who argue that it may be that there are even more very rare, but somewhat larger effect, alleles at work.
The Pith: There has been a long running argument whether Pygmies in Africa are short due to “nurture” or “nature.” It turns out that non-Pygmies with more Pygmy ancestry are shorter and Pygmies with more non-Pygmy ancestry are taller. That points to nature.
In terms of how one conceptualizes the relationship of variation in genes to variation in a trait one can frame it as a spectrum with two extremes. One the one hand you have monogenic traits where the variation is controlled by differences on just one locus. Many recessively expressed diseases fit this patter (e.g., cystic fibrosis). Because you have one gene with only a few variants of note it is easy to capture in one’s mind’s eye the pattern of Mendelian inheritance for these traits in a gestalt fashion. Monogenic traits are highly amenable to a priori logic because their atomic units are so simple and tractable. At the other extreme you have quantitative polygenic traits, where the variation of the trait is controlled by variation on many, many, genes. This may seem a simple formulation, but to try and understand how thousands of genes may act in concert to modulate variation on a trait is often a more difficult task to grokk (yes, you can appeal to the central limit theorem, but that means little to most intuitively). This is probably why heritability is such a knotty issue in terms of public understanding of science, as it concerns the component of variation in quantitative continuous traits which is dispersed across the genome. The traits where there is no “gene for X.” Additionally, quantitative traits are likely to have a substantial environmental component of variation, confounding a simple genotype to phenotype mapping.
Arguably the classic quantitative trait is height. It is clear and distinct (there aren’t arguments about the validity of measurement as occurs in psychometrics), and, it is substantially heritable. In Western societies with a surfeit of nutrition height is ~80-90% heritable. What this means is that ~80-90% of the variance of the trait value within the population is due to variance of the genes within the population. Concretely, there will be a very strong correspondence between the heights of offspring and the average height of the two parents (controlled for sex, so you’re thinking standard deviation units, not absolute units). And yet height is at the heart of the question of the “missing heriability” in genetics. By this, I mean the fact that so few genes have been associated with variation in height, despite the reality that who your parents are is the predominant determination of height in developed societies.
Since the beginning of this weblog (I’ve been writing for eight years) heritability has been a major confusion. Even long time readers misunderstand what I’m trying to get at when I talk about heritability. That’s why posts such as Mr. Luke Jostins‘ are so helpful. I had seen references to a piece online, The Causes of Common Diseases are Not Genetic Concludes a New Analysis, but I hadn’t given it much thought. Until Ms. Mary Carmichael’s post DNA, Denial, and the Rise of “Environmental Determinism”. She begins:
Michael Pollan, the well-known writer on food and agriculture, is a smart guy. His arguments tend to be nuanced and grounded in common sense. I like his basic maxim on nutrition – “Eat food. Not too much. Mostly plants” – so much that I recently promoted it in a Newsweek cover story. He’s the last person I’d suspect of reactionary thinking, which is why I wish I didn’t have to say this: Michael Pollan has made a deeply unfortunate mistake.
A few days ago, speaking to his 43,000 followers on Twitter, Pollan linked to an essay written by an environmental advocacy group that spends much of its time fighting the depradations of Big Agriculture. Curiously, the essay wasn’t about ecological destruction or even about agriculture. It was about human genetics. It argued that since genetics currently can’t explain everything about inheritance, genes must not influence the development of disease, and thus the causes of illness must be overwhelmingly environmental (meaning “uninherited” as opposed to “caused by pollution,” though the latter category of factors is part of the former one). This was a little like arguing that your engine doesn’t power your car because sometimes it breaks down in a way that confuses your mechanic — and concluding that gasoline alone is sufficient to make a car with no engine run. But Pollan took the argument at face value. He said it showed “how the gene-disease paradigm appears to be collapsing.” He was troubled that its contentions apparently had gone unnoticed: “Why aren’t we hearing about this?!”
Of course I had seen Dr. Daniel MacArthur’s post Bioscience Resource Project critique of modern genomics: a missed opportunity in my RSS, but when I started reading the rebuttal I immediately thought “Dr. Dan’s interlocutors sound kind of dumb,” and I stopped reading. After reading the post I don’t think they’re dumb, I think they’re being lawyerly. Much of the piece is a rhetorical tour de force in leveraging the prejudices and biases of the intended readership. This is the Intelligent Design version of Left-wing “Blank Slate” Creationism.* They smoothly manipulate real findings in a deceptive shell game intended to convince the public, and shape public policy. Their success is evident in Pollan’s response. “X paradigm appears to be collapsing.” “Why aren’t we hearing about this?” Does this sound familiar? Like Dr. MacArthur I think some of the criticisms within the piece are valid. Despite not being hostile to the maxim “better living through chemistry,” I do think that there has been an excessive trend toward pharmaceutical or surgical “cures” in relation to diseases of lifestyle (anti-depressants, gastric bypass, etc.). But we go down a very dangerous path when we make recourse to shoddy means toward ostensibly admirable ends. This sort of discourse is not sustainable! (just used a buzzword intended to appeal right there!)
I honestly can’t be bothered to say much more when so many others already have. This is a boat I missed. But if some of what I say above isn’t clear, I recommend you read the original essay. Then read Dr. MacArthur and Ms. Carmichael. If you’re hungry for more, Ms. Carmichael has a helpful list of links.
* Left Creationism had its most negative manifestation as Lysenkoism, but it suffuses the outlook of many who fear the emergence of a new Nazi abomination. Leon Kamin in the 1970s even claimed that IQ was not heritable at all! Though he backed off such an extreme position, it shows how confident he was that could claim such a thing.
Excellent post from Dr. Daniel MacArthur, Common copy number variation doesn’t explain much complex disease risk – but why not?:
The Wellcome Trust Case Control Consortium has just published the results of a massive survey of common, large DNA duplications and deletions (collectively termed copy number variation, or CNVs) in 16,000 patients suffering from complex diseases and 3,000 controls. The results come as no surprise, but are nonetheless disappointing: the study identified absolutely no novel CNVs associated with complex disease. Although three such variants were found to alter disease susceptibility, all three had been identified from previous studies.
The study’s findings suggests that – despite their size – common CNVs play very little role in the etiology of common, complex diseases like rheumatoid arthritis and type 2 diabetes, and researchers will have to look elsewhere to uncover the notorious “missing heritability” for these diseases.
Where to next? The field has already moved on with a new focus on rare variants, which (given the selection-based argument above) seem far more likely to yield useful findings. This year will see the launch of several very large studies taking a variety of approaches to dig into the lower end of the frequency spectrum: imputation using existing data-sets; new genome-wide association chips containing larger numbers of rare SNPs; and large-scale sequencing of candidate genes, whole exomes and even entire genomes. Rare variant discovery has already proved successful in the CNV field, and it seems likely that the next round of CNV association studies will prove enormously more fruitful than this study.
Missing heritability is a major issue. Though I guess it does science some good to have white whales to chase….