Mutations happen and must be accounted for in parentage investigations. In Chapter 6, we discussed the fact that mutation rates for STR loci are on the order of 1-4 per thousand meioses or 0.1-0.4% (see Table 6.3). The American Association of Blood Banks (AABB) has issued standards for parentage testing laboratories regarding mutations. The AABB standards recognize that mutations are naturally occurring genetic events and standard 5.4.2 states that the mutation frequency at a given locus shall be documented. Furthermore, standard 6.4.1 emphasizes that an opinion of non-paternity shall not be rendered on the basis of an exclusion at a single DNA locus (single inconsistency).
The 'two exclusion' rule is commonly accepted in parentage testing laboratories. In other words, if two genetic loci do not match between an alleged father and a child, the alleged father cannot be excluded as being the true biological father. It is important to keep in mind that the more genetic systems examined the greater the chance of a random mutation to be observed. With STR analysis often examining a battery of a dozen or more loci, it is not uncommon to see two inconsistencies between a child and the true biological father (Gunn et al. 1997, Nutini et al. 2003).
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