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STR Data Analysis and Interpretation for Forensic Analysts

Null Alleles

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A null allele is an allele that is present in a sample, yet is not amplified. A primer-binding site mutation can inhibit amplification for that allele and result in a null allele. If an individual is heterozygous and has a primer-binding site mutation for one of the alleles, the individual would type as a homozygote.


The manufacturers of various STR typing kits use different primer sets. If a DNA sample has a mutation in a primer-binding region specific to kit A, but no mutation in the primer-binding region specific to kit B, a rare discordance in allele calls can occur when comparing typing results produced by these two manufacturers' kits.07

When comparing DNA typing results from different kits, null alleles (due to primer-binding site mutations) can result in discrepant DNA types at a particular locus. It is important to understand that although null alleles are rare, they must be considered when interpreting potential matches.

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