Mutated alleles expressing allelic heterogeneity can be classified as adaptive or disadaptive. The mitochondrial allelic mutations are inherited maternally. Each phenotype is based upon the allelic variation within the individual genotype. The gene is associated with allelic variants. Gene conversion may be allelic or ectopic. The second type of data is changes in allelic frequencies or phenotypes across different generations. This is an example of allelic genetic heterogeneity. Genetic drift is a cause of allelic frequency change within populations of a species. Real-time PCR is useful in pathogen detection, gene expression and regulation, and allelic discrimination. However, it also depends on the allelic status of the genes undergoing recombination. VNTRs are the class of clustered tandem repeats that exhibit allelic variation in their lengths. This is sometimes called allelic complementation, or interallelic complementation. Other studies have shown how human-induced selection is responsible for the allelic variation in pigs. SNP arrays help scientists identify patterns of allelic imbalance. The third type of data is differences in allelic frequencies across space.