Forensics: DNA, Decomposition and Time Since Death

A DNA profile examines variable regions and compares fragment patterns or sequence-derived measurements. PCR can amplify selected regions, and gel electrophoresis separates charged DNA fragments, with smaller fragments generally moving farther through the gel under the same conditions. Use controls and size markers. Contamination can introduce additional DNA; a mixed sample is not automatically evidence that one person contributed every band. A DNA profile is a selected comparison, not a picture of all genes.

Time since death may be estimated using body cooling, muscle changes, decomposition and insect development or succession. Each method depends on conditions such as ambient temperature, access by insects, clothing and body size. Blowfly stage can help estimate a minimum interval since colonisation, which need not equal the exact time of death. A reliable answer explains assumptions and uses multiple independent observations rather than presenting one clock-like process as exact.

A matching profile can make an association more likely, but interpretation depends on population frequencies, sample quality and the possibility of contamination or related contributors. A mismatch at a reliably measured locus may exclude a candidate under the model. Keep the biological observation separate from the legal or historical conclusion. Numerical probabilities need their assumptions stated; a rare match is not automatically the probability that a particular person is guilty.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.