Reference material for this review was selected on the basis of its relevance for specifically addressing the effects, outcomes, or effect of population stratification on allelic association studies. We used our own reference compilations and PubMed to identify the references cited in this work. Beyond our own material, our search terms included “population stratification”, “admixture”, “spurious association”, “genomic control”, and “pharmacogenetic association”. For inclusion, recent
ReviewPopulation stratification and spurious allelic association
Section snippets
Genetic association studies
Statistical evidence for an association between an allele and a phenotype comes from one of three situations.39 First, the allele itself might be functional and directly affect expression of the phenotype. Second, the allele might be correlated with, or be in linkage disequilibrium with, a causative allele located nearby. Third, the association could be attributable to chance or artifact—eg, confounding or selection bias.
Many study designs are available for association analyses, which can be
Treatment of population stratification in association studies
The problem of population stratification can be viewed essentially as one of sample matching. In general, for any well-designed epidemiological case-control study, the source population from which controls are sampled should be that from which cases are also sampled.21 Population stratification can arise when the genetic background of the source populations differs between cases and controls.
One obvious solution to the difficulty of stratification is to carefully match cases and controls on the
Controlling for stratification with families
The most widespread study design for genetic matching includes use of relatives as controls. There are many family-based matching designs and corresponding statistical methods for discrete and continuous traits.46, 62, 63 The most popular method, and that from which most others are derived, is the transmission-disequilibrium test (TDT).64, 65 The TDT design requires an affected individual and his or her parents, and uses the mendelian principle that for any polymorphic marker, each parent
Controlling for stratification with anonymous genetic markers
There are several methods that protect against population stratification-related drawbacks but do not need family samples. Pritchard and Rosenberg51 popularised the notion of using anonymous genetic markers scattered throughout the genome as indicators of the amount of background diversity in cases and controls. They reasoned that as long as the markers were independent of those affecting the disease of interest, and largely did not correlate with each other, they should reflect baseline
Implications for pharmacogenetic studies
An expanding area of interest in application of SNPs to investigations of disease pathophysiology is stratification of populations by their genetically determined response to therapeutic drugs (pharmacogenetics).83 Ideally, one would like to be able to stratify a population needing treatment into those likely, or unlikely, to respond to treatment and those likely, or unlikely, to have adverse side-effects. One of the primary goals of pharmacogenetics is to understand the role that sequence
Conclusions
Failure to replicate genetic association studies is a genuine concern,9, 34, 44 yet more often it involves poor study design and execution—in particular an absence of appreciation for the sample sizes needed to detect modest genetic effects and overinterpretation of marginal results—than undetected population stratification. For most complex human diseases, the reality of multiple disease-predisposing genes of modest individual effect, gene-gene interactions, gene-environment interactions,
Search strategy and selection criteria
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