Background: DNA sequence variation including base(s) changes and insertion or deletion in the primer binding region may cause a null allele and, if this changes the length of the amplified fragment out of the allelic ladder, off-ladder (OL) alleles may be detected.
Aim: In order to provide accurate and reliable DNA evidence for forensic DNA analysis, it is essential to clarify sequence variations in prevalently used STR loci.
Subjects and methods: Suspected null alleles and OL alleles of PlowerPlex16® System from 21,934 unrelated Chinese individuals were verified by alternative systems and sequenced.
Results: A total of 17 cases with null alleles were identified, including 12 kinds of point mutations in 16 cases and a 19-base deletion in one case. The total frequency of null alleles was 7.751 × 10(-4). Eight hundred and forty-four OL alleles classified as being of 97 different kinds were observed at 15 STR loci of the PowerPlex®16 system except vWA. All the frequencies of OL alleles were under 0.01.
Conclusion: Null alleles should be confirmed by alternative primers and OL alleles should be named appropriately. Particular attention should be paid to sequence variation, since incorrect designation could lead to false conclusions.
Keywords: Chinese population; OL allele; STR loci; null allele; sequence variation.