This note explores the effect of within-strain sample sizes on the correlations between a phenotype and a molecular-genetic marker in a battery of inbred strains. It is shown that the maximum correlation possible between a molecular marker and a behavioral or neuronal phenotype equals the additive-genetic correlation. How close the strain correlation will approach the additive-genetic correlation depends only on heritability and within-strain sample sizes. The equations derived can be used to optimize designs of studies attempting to localize Quantitative Trait Loci utilizing Recombinant Inbred Strains, provided information about the heritability of the character under study is available.