16 – To Investigate Patterns Using a DNA Profile (LC 2027) (Leaving Cert Biology): Revision Notes
📚 Revision Notes
16 – To Investigate Patterns Using a DNA Profile
DNA profiling is a powerful molecular technique that allows scientists to identify individuals and establish biological relationships by analysing their unique genetic fingerprints. This investigation explores how DNA profiles can be used to solve crimes and determine paternity through pattern recognition in gel electrophoresis results.
What is DNA profiling?
DNA profiling, also known as DNA fingerprinting, creates a unique genetic pattern for each individual based on specific regions of their DNA. These patterns appear as bands on a gel when DNA samples are processed through electrophoresis. Each person's DNA profile is unique (except for identical twins), making it an invaluable tool for identification purposes.
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The technique works by examining short tandem repeats (STRs) - sections of DNA where short sequences are repeated different numbers of times in different people. When DNA is cut with enzymes and separated by gel electrophoresis, these variations create distinctive band patterns.
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Each person's DNA profile is completely unique (except for identical twins), which is what makes DNA profiling such a reliable method for identification and relationship determination.
Understanding gel electrophoresis results
In DNA profiling, gel electrophoresis separates DNA fragments by size, with smaller fragments travelling further through the gel than larger ones. The resulting bands appear as dark lines on the gel, with each band representing DNA fragments of a specific size. When comparing samples, scientists look for matching band patterns between different DNA profiles.
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The key principle of electrophoresis is that DNA fragments migrate through the gel at different rates based on their size - this creates the distinctive banding patterns that scientists use for comparison.
Crime scene analysis
DNA profiling plays a crucial role in forensic investigations by allowing scientists to compare DNA evidence from crime scenes with DNA samples from suspects. In criminal cases, investigators collect DNA samples from the crime scene and compare them with profiles from potential suspects.
When analysing crime scene DNA, scientists examine whether any suspect's DNA profile matches the evidence collected. A match occurs when the band patterns are identical between the crime scene sample and a suspect's DNA. If no bands match between a suspect and the crime scene evidence, that person can be eliminated as a source of the DNA.
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Worked Example: Crime Scene DNA Analysis
Step 1: Collect DNA sample from crime scene
Step 2: Prepare DNA profiles from crime scene sample and all suspects
Step 3: Compare band patterns systematically
Step 4: Look for identical band positions between samples
Step 5: If bands match exactly → potential match identified
Step 6: If no bands match → suspect eliminated
The process involves preparing DNA profiles from both the crime scene sample and samples from all suspects, then carefully comparing the band patterns. Any differences in band position or presence indicates that the DNA samples came from different individuals.
Paternity testing using DNA profiles
DNA profiling is highly effective for determining biological parentage because children inherit half their DNA from each parent. This means a child's DNA profile will contain bands that match both the mother's and father's profiles.
Analysing parent-child relationships
In paternity testing, scientists compare the child's DNA profile with the mother's profile and potential fathers' profiles. The biological father will share DNA bands with the child that don't come from the mother. Specifically, scientists look for bands in the child that match the potential father but not the mother.
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Worked Example: Paternity Analysis Process
Step 1: Examine child's DNA profile and identify all bands
Step 2: Compare child's bands with mother's profile
Step 3: Identify which bands came from the mother
Step 4: Determine which bands in child are not from mother (paternal bands)
Step 5: Compare paternal bands with potential father's profile
Step 6: The man whose profile contains the matching paternal bands is the biological father
The methodology involves examining each band in the child's profile and determining whether it came from the mother or father. Bands that appear in the child but not in the mother must have come from the biological father. Therefore, the man whose DNA profile contains these matching paternal bands is identified as the biological father.
Multiple paternity scenarios
When testing multiple potential fathers, scientists compare each man's DNA profile with the child's profile, accounting for the bands already identified as maternal. The potential father whose profile shows the best match with the child's non-maternal bands is determined to be the biological father.
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This process requires careful analysis because some bands may appear in multiple individuals by coincidence, but the overall pattern of matching bands will clearly identify the biological father.
Key principles of DNA analysis
Several important principles guide DNA profiling analysis:
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Fundamental Principles of DNA Analysis:
- Uniqueness: Each person's DNA profile is unique (except identical twins)
- Inheritance: Children inherit DNA from both parents in equal proportions
- Band matching: Identical band patterns indicate DNA from the same source
- Pattern comparison: Systematic comparison reveals relationships and identities
- Secondary data: Analysis often uses existing DNA profile data rather than fresh samples
The reliability of DNA profiling makes it one of the most trusted methods for both forensic identification and paternity determination. The technique's accuracy depends on proper sample collection, laboratory procedures, and careful analysis of the resulting band patterns.
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Key Points to Remember:
- DNA profiles create unique patterns of bands that can identify individuals and establish biological relationships
- Crime scene analysis involves comparing DNA evidence with suspect profiles to find matches or eliminate suspects
- Paternity testing works by identifying which potential father shares DNA bands with the child that don't come from the mother
- Children inherit half their DNA from each parent, creating predictable patterns in family DNA profiles
- Careful comparison of band patterns is essential for accurate interpretation of DNA profiling results