Estimation of mutational events at a DNA marker may be achieved by comparison of genotypes from offspring to those of their parents. Genotype data from
Until recently, only general information on STR mutation rates was reported -namely, how many mutations occurred relative to the number of meioses measured (see Table 6.3). The realization that certain alleles are more prone to mutation than others has prompted the American Association of Blood Banks (AABB) to carefully examine which alleles were mutating based on records from accredited parentage testing laboratories.
Appendix 5 in the AABB Annual Report Summary for Testing in 2002, prepared by the parentage testing program unit in November 2003, notes the number of paternal and maternal mutations by both locus and allele. For example, with the STR locus FGA an apparent change from allele 24 to 25 was observed 62 times (11.7%) out of 530 total paternal mutations seen in 2002, while an apparent change from allele 19 to 20 was seen only eight times (1.5%). In general longer alleles were seen to mutate more frequently.
The directionality of the mutation as either an expansion or a contraction of the repeat array can also vary significantly. For example, with paternal D16S539 mutations observed in 2002 there were 10 instances of allele 11 expanding to become allele 12 but only four examples of allele 11 contracting to allele 10. The process of expansion and contraction of the STR repeat regions probably occurs in a similar fashion as illustrated in Figure 6.2 for stutter product formation.
As this information continues to be collated in future studies, it should prove useful in refining mutation rates and aid in a better understanding of the process of STR origins and variability over human history.
American Association of Blood Banks Annual Report Summary for Testing in 2002
D.N.A. Box 6.1 Allele specific mutation rates paternity trios involving a father, a mother, and at least one child is examined. A discovery of an allele difference between the parents and the child is seen as evidence for a possible mutation (Figure 6.10). The search for mutations in STR loci involves examining many, many parent-child allele transfers because the mutation rate is rather low in most STRs.
The majority of STR mutations involve the gain or loss of a single repeat unit (see D.N.A. Box 6.1). Thus, a VWA allele with 14 repeats would show up as a 13
Mutational event observed in family trios. Normal transmission of alleles from a STR locus (a) is compared here to mutation of paternal allele 14 into the child's allele 13 (b).
or a 15 in the next generation following a mutational event (Figure 6.10). Paternal mutations appear to be more frequent than maternal ones for STR loci (Sajantila et al. 1999, Henke and Henke 1999). However, depending on the genotype combinations it can be difficult to ascertain from which parent the mutant allele was inherited.
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