Murder Forensics

In this project, we were tasked with solving a murder given a variety of evidence and a brief summary of the event. To collect and analyze this evidence, we had to use a variety of forensics techniques and some biology concepts to draw a cohesive conclusion about who committed the crime and for what reason. After we had collected all data and performed our analyses, we presented our findings to the class who had also drawn their own conclusions from the same data.

In this project, we learned about each technique for a period of time followed by execution of the said method. We learned about how DNA fingerprinting works, how blood types work, how to construct a pedigree, what karyotypes are, how fingerprints are controlled by genes, how to perform chromatography, and how genetic inheritance of traits and conditions work.

The Presentation

Copy of Forensic Crime Scene Project

Core Concepts

Forensics

Forensics is the use of scientific methods to aid in the solution or detection of a crime. Forensics can often link a person to a specific object, place, or time through a variety of methods. However, this does not mean that forensics instantly identifies the perpetrator of a crime — a person can potentially be, say, at the crime scene but not guilty in any way.

Fingerprints

Fingerprints are a common tool in forensics. Each person has a unique set of fingerprints (which may be the same on all fingers or even different between fingers) that matches those of no other person. Because human fingers are somewhat oily, when we touch an object the ridges on our fingers (our fingerprints) become outlined in trace amounts of oil on the object. This means that our fingers act like "stamps" everywhere we touch, so through a process known as dusting we can make these traces of oil visible and detect where a specific person has touched.

By dusting a little bit of powder over the potential fingerprint area, the powder adheres to the places where the oil and moisture is, allowing the fingerprint to be visibly traced by the powder. Then, it can be compared to the fingerprints of the known suspects, and whichever one is closest will be the person who touched that object. There are three general types of fingerprints — arches, loops, and whorls (see below). Once the type is nailed down, within the type small variations in aspects can finish identifying the specific person.

Polarity

In the context of chemistry, polarity refers to the chemical charge of a molecule. Because of how charges attract and repel each other, putting many molecules with different polarities next to each other can cause them to arrange themselves in a specific pattern.

Chromatography

Chromatography is the process of separating out the different pigments contained in an ink sample. In this investigation, it was used to determine which pen wrote the murderer's note. While many types of pens may look identical when written with (i.e. a dark black or dark purple), they are often composed of many different inks that provide a unique formulation of their color. When these inks are separated, it is then possible to identify exactly which pen was used to create the given ink sample, even if the pens all "look" the same.

The chromatography we performed relied on using Isopropyl Alcohol (IPA) to carry the ink from a small dot up a sheet of paper through capillary action. The different inks, with their different chemical polarities, naturally sorted themselves out as the IPA progressed higher and higher. This created a spectrum that could then be easily distinguished (see the slideshow above).

Inheritance

Inheritance is the process of how traits are passed down from generation to generation. Each generation, an individual may end up with one of many types of a specific trait — for example, for eye color an individual may have brown or blue eyes (etc). Each of those potential "options" is called an allele for the trait. The physical characteristic of an allele is the phenotype — the genetic code that encodes for that allele is the genotype.

Simple inheritance involves dominant and recessive alleles. When you receive genetic information from your parents, you get one allele from each of your parents: that's two possibly conflicting alleles! The mechanism of dominance solves this issue by making some alleles dominate other recessive ones. For example, in eye color, the brown eye allele is dominant while the blue eye allele is recessive. That means that if you get a brown allele from one parent but a blue allele from another parent, you'll still end up with brown eyes since brown is dominant. The only way to get blue eyes is to get a blue allele from both of your parents.

Things get more complex when there isn't an easy "win-lose" dominance situation. Incomplete dominance is when a mix of the two alleles ends up creating a mix of the two phenotypes — for example, when a certain type of flower has a red-petalled and a white-petalled parent, the flower ends up blossoming with pink petals instead of either red or white. Co-dominance is when a mix of the two alleles creates a new result entirely, such as when a white and red cow produce a roan (splotched) offspring.

There are a few final terms and tools to make this all easier. A homozygous genotype is one which has two of the same alleles (i.e. when both parents pass down the same allele). Homozygous dominant means that the genotype is two of the dominant allele, while homozygous recessive means that the genotype is two of the recessive allele. A heterozygous genotype is one which has two different alleles, like the pink flower example from earlier (or a brown-eyed person with just one blue-eyed allele). When notating this, a capital letter is used to denote a dominant allele and a lowercase letter is used to denote a recessive one. The genotype for a person with one blue-eye allele and one brown-eye allele could thus be denoted Bb if we chose "B" to mean brown and "b" to mean blue. The dominant allele always comes first.

We can also use "Punnett Squares" to enumerate all possible genotypes given two parents to 'cross'. See the example below for a banana where green (G) is the dominant color.

Sexually-linked Traits

Some traits are linked to the sex chromosomes, X and Y. The sex of the offspring itself is expressed through a combination of the X and Y chromosomes; a male offspring receives an X from the mother and Y from the father giving it the chromosomes XY, while a female offspring receives an X from the mother and an X from the father to make XX. Thus, if a recessive allele on the X chromosome corresponds to a disease (case in point: hemophilia), then females have a lower chance of having the disease since a "working" (dominant) allele on either of their X chromosomes will negate it while just 1 recessive allele on the male's only X chromosome will lead to it being expressed. Importantly, females can carry a disease by having the recessive allele on only one of their X chromosomes. They will not suffer from the condition, since the dominant allele on the other X overrides the recessive one, but they can potentially pass it on to their offspring if that X chromosome is the one passed on.

Blood Types

There are four (main) blood types — A, B, AB, and O. A and B are dominant, while O is recessive and AB is the co-dominant type. Since each person has just one blood type, blood can then be determined to have either:

Definitely not come from a certain person (blood types don't match), or
Potentially come from a certain person (blood types match)

The reason that even a match doesn't mean the blood came from the person is that there are only 4 (8 counting negative/positive for the Rh protein) blood types and thus many, many people in the world have the same blood type. Therefore, finding blood on the crime scene whose type matches one of the suspects does not make them guilty (or even show that they were there), it only shows that they potentially lost blood at the scene. A find of blood on the scene that does not match, however, does indicate that they did not lose blood at the scene with 100% certainty.

Pedigree

A pedigree is a chart showing the "family tree" of a group of people as well as any genetic conditions that they may have carried or expressed. This allows an investigator to track the propagation of a genetic condition through a bloodline and thus be able to draw conclusions if a sample taken at the crime scene indicates a genetic condition. See the slideshow above for an example of a pedigree chart.

Karyotype

A karyotype is an image which shows all of a person's chromosomes. (Usually, this is 23 pairs, but conditions may alter this). A karyotype can easily show potential conditions like having too many or too few of specific chromosomes, but it can also indicate finer differences in chromosomes themselves (different DNA). By comparing karyotypes of different people, it can be determined if an unknown karyotype matches any known ones and thus determine who an unknown person is from their karyotype sample. A mock-up karyotype is included in the slideshow above.

Gel Electrophoresis (DNA Fingerprinting)

Each person has their own unique DNA. Since DNA contains a huge amount of data, it is impractical to directly compare each base in two strands of DNA (such as a known person and an unknown person to be identified). Therefore, a method to approximate equality can be very beneficial — such a method is gel electrophoresis. The basic principle is that everyones' DNA contains specific "key codes" (say, AGGC) that are repeated over and over, but because each person's DNA is unique these codes are placed at different distances from each other on the strand between people. Thus, by applying a restriction enzyme that cuts up a DNA sample at these specific codes, two samples of DNA can be converted into two groups of different-lengthed snippets (or the same lengths if the two DNA samples were matches). Gel electrophoresis then involves inserting those DNA snippets into a gel and then applying a constant electrical charge through the gel. This separates and arranges the snippets by their size, and after a visualizing dye is applied (making the DNA strands visible) the arrangement of these snippets can be easily seen. Then, it is only a quick matter of comparison to determine whether the two DNA samples match up, since matching DNA will have groups of snippets in the exact same places. While we did not perform electrophoresis on true DNA samples, mock-up electrophoresis on various dyes can be seen in the slideshow.

Reflection

This was a lengthy project and involved many different skills. Therefore, this project was most certainly a critical thinking-based success. Our group was able to manage all of the different sources of evidence effectively to create a single cohesive conclusion, and to explain why outliers might appear to exist. We were also able to collaborate very well, with each person managing their own section of the investigation and then bringing everything together to compare our results. We were able to all add our own input to the project and work collectively to decide what the best conclusion was. Finally, as alluded to, our conscientious learning was very strong. Because of our divide-and-conquer approach, we were able to essentially complete our entire presentation on the first work day, while many other groups took several days to complete theirs. We never fell behind throughout the entire process and worked hard when we needed to stay on top of it.

There were a few minor issues, such as our communication. The presentation was good, but not entirely smooth, as one of our group members did not show up to class that day until just before the presentation. While she had attempted to notify us of this through email, we did not make the effort to identify this and were thus caught unaware when she appeared to present. This could have been avoided with better communication, or rather better acceptance of communication. Our conscientious learning wasn't flawless either — occasionally we were uncertain of what to do and did not utilize the time, which we could have used to get even further on top of the time. This was, however, not much of an issue because of our consistent pace for moving forward.

This project was, therefore, nearly perfect. There were no major issues and we clearly executed the task with good understanding of the content covered. It was thus another success in many areas of learning.

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