Investigation of Mosaicism Detected in STR Typing

Abstract

Short tandem repeats (STRs) are the most popular markers for human identification in forensics. These markers can be easily analyzed through a multiplex polymerase chain reaction and electrophoresis and provide high discrimination power. However, in STR analysis, several atypical phenomena can be observed such as allelic dropouts, drop-ins, or imbalance, which may be due to DNA polymerase slippage or DNA degradation effects. The observed atypical STR profiles can also provide information for mixed DNA samples or chromosomal abnormalities. In this study, we report a case of mosaicism detected in routine casework of paternity testing. Hair samples from a phenotypically normal male were tested, and the result presented a typical STR profile of a female for the amelogenin gene (XX). Through chromosome analysis using peripheral blood, it was found that 45,X/46,XY mosaicism resulted in the discrepancy between the genotype and the phenotype. In addition, the amount of Y chromosome detected was particularly low in hair compared to that in blood. This study shows that mosaicism can make interpretation difficult during STR analysis and suggests that sample types and repeated analysis should be considered even for routine STR testing.