Tall fescue contains three genomes (PPG1G1G2G2) and is an allohexaploid (2n = 6x = 42), meaning that the six sets of chromosomes present in tall fescue have been derived from different ancestor species (see Chapter 2). The P genome is derived from the diploid (2n = 2x = 14) progenitor meadow fescue [Lolium pratense (Huds.) Darbysh. = Festuca pratensis Huds.] while the G1G2 genome is from tetraploid (2n = 4x = 28) fescue [(4x) (L. arundinaceum = F. arundinacea var. glaucescens Boiss.) (Xu et al., 1991). The tall fescue genome contains more than 5 billion pairs of bases, or approximately 5.27 to 5.83 × 106 kilobases (kb) (Seal, 1983). Only a portion of this DNA contributes to gene expression. The large genome size and the presence of a large number of nonexpressed DNA sequences makes identifying and selecting genes of interest very challenging. Molecular markers and maps have been developed to facilitate this selection process in tall fescue.