Estimation of mutation probabilities for autosomal STR markers

It is well known in forensic genetics that mutations on STR loci need not lead to genetic inconsistencies between the genotypes of parents and children. In this article we look at a generalization of this phenomenon of “hidden” mutations: we investigate the probability with which a mutation of d repeat units appears to be one of k<d repeat units, in the sense that a mutation of k repeat units is sufficient to explain the observed genotypes. The case k=0 then corresponds to a hidden mutation. We give tables of these probabilities for many commonly used STR markers and show how they can be used to obtain actual mutation rates from apparent mutation rates per locus, mutational distance and gender. We illustrate the difference between apparent and actual mutation rates by using mutation counts from the AABB's annual reports of 2003 and 2008. These computations suggest that all mutations, but especially those of distance greater than one, are more common than counts of apparent distances suggest.