Influence of genotype span on mutation rate. Span is defined as the difference in allele length, measured in repeat units, of alleles in the parental genotype in which a mutation was identified. For each mutation, the expected span was calculated by averaging across all other alleles of that length in the dataset (open circles). Data from 203 observed mutations are presented as filled circles ± 1 standard error. For clarity, the graph is truncated to include only repeat numbers where three or more mutations were recorded, thereby excluding 12 of the original 215 mutations (span range 0 – 16). Considering all 215 mutations, mean span of mutated alleles is significantly greater than expected by chance (one sample t-test, t = 3.02, p = 0.0028, two-tailed).