LDheatmap: An R Function for Graphical Display of Pairwise Linkage Disequilibria Between Single Nucleotide Polymorphisms. We describe the R function LDheatmap() which produces a graphical display, as a heat map, of pairwise linkage disequilibrium measurements between single nucleotide polymorphisms within a genomic region. LDheatmap() uses the grid graphics system, an alternative to the traditional R graphics system. The features of the LDheatmap() function and the use of tools from the grid package to modify heat maps are illustrated by examples.