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STR Data Analysis and Interpretation for Forensic Analysts

Stochastic Effects

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Stochastically induced heterozygote imbalance may be observed in STR analysis due to the effective low copy number of DNA templates in degraded DNA.

The amplification process can produce many copies from a relatively low quantity of DNA. If too small a quantity of DNA is introduced at the beginning of amplification, it is possible that heterozygous alleles may amplify differentially. The first few cycles of the amplification process are extremely important; if imbalanced amplification occurs, it will result in stochastic effects.

An analyst can assess the data by calculating the heterozygosity of alleles. If the heterozygote balance is less than 70%, this could indicate a mixture and/or stochastic amplification. Laboratories evaluate the threshold at which these stochastic effects may be more prevalent. This stochastic threshold is used by the scientist to evaluate when alleles of a heterozygote pair may not be detected and plays an important role in the interpretation of DNA profiles.

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