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The stepwise mutation model (SMM) was introduced to population genetics theory by Ohta and Kimura (1973). It was based heavily the idea from Bulmer (1971) that regularities existed in the frequencies of alleles that could be determined by electrophoresis (was later determined unfit to analyze electrophoresis alleles). Bulmer stated it was a model of mutation in which alleles would increase or decrease their net charge by one unit, which would move the allele from one mobility class to another(Valdes, Slatkin & Freimert 1993). Under this hypothesis, alleles in the same class would not necessarily be identical by descent but only identical in state. The assumption underlying the stepwise mutation model is that repeat number changes by only one or two as a result of mutation. Given this assumption, the model is sometimes suited for analysis of microsatellite loci. The SMM allows for various summary statistics such as: quantifying heterozygosity, allele frequencies, number of alleles, and variance in allele size.

Valdes, A.M., Slatkin, M. & Freimert, N.B. (1993) Allele Frequencies at Microsatellite Loci: The Stepwise Mutation Model Revisited. Genetics Society of America, 749, 737–749.