During STR analysis part of the lab, PCR was performed on the DNA that was isolated from a small quantity of dried skin cells that were on the murder weapon. The crime scene and suspect's DNA was amplified at the TH01 locus and the amplified DNA was ran on a 3% agarose gel and viewed using the GelDoc (Kluftinger, J. and Punket, R., 2015). Upon analysis it can be determined if one of the suspects was present at the crime scene. The results of the STR analysis results can be seen in table 3.
Table 4. The short tandem repeat analysis that ran on 3% agarose gel. The genotype at the TH01 locus was determined from the agarose gel and the allele size of each band was determined in base pairs. From the base pairs the allele frequency could be determined from the known allele frequencies for the base pairs. The random match probability (RMP) calculation is based on the Hardy-Weinberg equilibrium equation, p^2 + 2pq + q^2 = 1. From the Hardy-Weinberg equilibrium equation, the heterozygote and homozygote locus RMP formulas can be described as, 2pq and p^2 or q^2, respectively.
Suspect |
Genotype at TH01 Locus |
Allele size (bp) first band |
Allele Frequency |
RMP |
CS |
5, 8 |
179 191 |
0.019 0.1255 |
0.00477 |
A |
5, 9 |
179 195 |
0.019 0.1689 |
0.00642 |
B |
4, 5 |
175 179 |
0.019 0.019 |
0.00072 |
C |
5, 8 |
179 191 |
0.019 0.1255 |
0.00477 |
D |
4, 9 |
175 195 |
0.019 0.1689 |
0.00642 |
From table 3 we can see that the genotype at locus TH01 at the crime scene is 5, 8 and that suspect C has the same genotype at the TH01 locus as the crime scene. That means that suspect C was was present at the crime scene because their DNA was collected from the scene.