The U.S. Department of Justice has established a database that catalogs \(P C R\) amplification products from short tandem repeats of the \(Y\) (Y-SIRs) chromosome in humans. The database contains polymorphisms of five U.S. ethnic groups (African Americans, European Americans, Hispanics, Native Americans, and Asian Americans) as well as worldwide population (a) Given that STRs are repeats of varying lengths, for example \((\mathrm{TCTG})_{9-17}\) or \((\mathrm{TAT})_{6-14}\) explain how PCR could reveal differences (polymorphisms) among individuals. How could the Department of Justice make use of those differences? (b) \(Y-S T R s\) from the nonrecombining region of the \(Y\) chromosome (NRY) have special relevance for forensic purposes. Why? (c) What would be the value of knowing the ethnic population differences for \(Y\) -STR polymorphisms? (d) For forensic applications, the probability of a "match" for a crime scene DNA sample and a suspect's DNA often culminates in a guilty or innocent verdict. How is a "match" determined, and what are the uses and limitations of such probabilities?

Step by step solution

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a) Understanding Polymerase Chain Reaction (PCR) and its application

PCR is a technique used to amplify a specific DNA sequence, producing millions of copies. It can reveal differences (polymorphisms) among individuals by amplifying the short tandem repeats (STRs) in their DNA. These polymorphisms can be observed by analyzing the lengths of amplified DNA fragments. The Department of Justice can use these differences to distinguish between individuals and help in identifying potential suspects in criminal cases by comparing DNA samples found at crime scenes with those stored in the database.

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b) Relevance of Y-STRs from the NRY region for forensic purposes

Y-STRs are present in the nonrecombining region of the Y chromosome (NRY), which means that they do not undergo recombination during the formation of sperm cells. This makes them particularly relevant for forensic purposes because Y-STR patterns are passed from father to son with minimal alterations. Consequently, Y-STRs can be used in cases where biological evidence contains exclusively male DNA, for example, in sexual assault cases, as well as in paternity testing and tracing male lineage in genealogical studies.

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c) The value of knowing ethnic population differences for Y-STR polymorphisms

Knowing the ethnic population differences for Y-STR polymorphisms can help increase the accuracy and relevance of forensic DNA analysis. Different ethnic groups may have distinctive Y-STR patterns, which can aid in narrowing down the pool of potential suspects, providing valuable context in investigations. Additionally, understanding ethnic distribution of Y-STR polymorphisms can facilitate the improvement of the statistical analysis models used in calculating the probability of a DNA match, further enhancing the reliability of forensic evidence.

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d) Determining a DNA "match" and its uses, limitations

A DNA "match" is determined by analyzing and comparing specific regions of DNA in two samples, such as an evidence sample found at a crime scene and a suspect's DNA sample. If the DNA profiles from the two samples are identical at the examined regions, a match is declared. The forensic investigators then calculate the probability of this match occurring by chance, considering the frequency of the observed DNA profile in the general population. Though DNA matches have significant value in criminal investigations, it is essential to acknowledge their limitations. If a DNA match is found, it does not mean the suspect is guilty but rather that the suspect should be investigated further. False matches can occur due to laboratory errors, cross-contamination, or the presence of identical twins. Additionally, the absence of a DNA match does not necessarily mean the suspect is innocent; it could indicate that evidence was lost or not found. It is crucial to consider the context and corroborate DNA evidence with other types of evidence to ensure a reliable verdict.

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