Population genetics is the study of the frequency of occurrence of alleles within and between populations. Frequency information can be applied to a variety of population issues such as understanding the genetic basis and probabilities for disease transmission, developing breeding programs for endangered or agricultural species, and elucidating the evolutionary history of a species.

Traditionally, the study of population genetics involved the identification of different alleles through observation of the expressed traits or outward, physical manifestation of a gene, called the phenotype . Mendelian genetics allowed population geneticists to identify the heritable form of a gene ( genotype ) including individual variants (alleles). Advances in molecular genetics - laboratory research on the chemical structure encoding the genes (DNA) - facilitated identification of single genes at the molecular or biochemical level. Regardless of the method used to identify genes and their alleles, population geneticists use statistical analyses of allele frequencies to understand and make predictions about gene flow in populations past, present, and future.

Note: The following discussion is confined to diploid, sexually reproducing organisms; however, there are comparable sets of statistics applicable to organisms with other reproductive strategies and life cycles.