Value Of Highly Polymorphic Markers In Deciphering Mixtures

The probability that a mixture will be detected improves with the use of more loci and genetic markers that have a high incidence of heterozygotes. The detectability of multiple DNA sources in a single sample relates to the ratio of DNA present from each source, the specific combinations of genotypes, and the total amount of DNA amplified. In other words, some mixtures will not be as easily detectable as other mixtures.

Using highly polymorphic STR markers with more possible alleles translates to a greater chance of seeing differences between the two components of a mixture. For example, D18S51 has 51 possible alleles while TPOX only has 15 known alleles (see Appendix I) making D18S51 a more useful marker for detecting mixtures. Likewise, the more markers examined (e.g., by using a multiplex STR amplification), the greater the chance to observe multiple components in a mixture.

The quantity of each component in a mixture makes a difference in the ability to detect all contributors to the mixed sample. For example, if the two DNA sources are in similar quantities they will be much easier to detect than if one is present at only a fraction of the other. The minor component of a mixture is usually not detectable for mixture ratios below the 5% level or 1:20 (Cotton 1995, Applied Biosystems 1998). The AmpFlSTR kits recommend that the minor component be above 35 pg in quantity to obtain a reliable genotype result (Applied Biosystems 1998).

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