= Linkage-disequilibrium marker tagging
An observed <genetic marker> tags an unobserved causal <genetic variant> when <linkage disequilibrium> correlates their allele indicators. Their squared <allelic correlation> $r^2$ measures prediction strength. Under standard local-alternative score-test approximations, using the marker rather than the causal allele reduces the noncentrality parameter by roughly $r^2$, suggesting a sample-size multiplier about $1/r^2$. This is an approximation requiring the same disease model and sampling design, not a universal power identity.
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