Novel genetic variants of the G Protein-Coupled Receptor 4 (GPR4) gene are described. Various genotypes, haplotypes, and haplotype pairs that exist in the general United States population are disclosed for the GPR4 gene. Compositions and methods for haplotyping and/or genotyping the GPR4 gene in an individual are also disclosed. Polynucleotides defined by the sequence the haplotypes disclosed herein are also described.