Chapter 3: Forensic Biology: A Subdiscipline of Forensic Science

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You know, when a detective tapes off a crime scene, they're not just looking for obvious clues like weapons.

They're gathering these tiny, almost invisible, silent witnesses.

Myological ones.

Exactly.

Like blood, hair, saliva.

Evidence that, well, it's a piece of science, isn't it?

But how does that tiny speck, maybe a faint stain on the pavement, actually become solid proof in court?

Well, that evidence only speaks if the whole forensic system behind it is solid.

And today we're doing a deep dive into that world,

specifically, forensic biology.

We're using Richard Lye's fantastic book, Forensic Biology, Second Edition, as our guide.

The idea is to pull out the core concepts, the definitions, the history, so you can really grasp how biological evidence gets analyzed in the justice system today.

So our mission is basically to get a quick handle on the key terms, the timeline, and the current goal standards.

The science that makes it all work in modern investigations.

And maybe start with the system itself, because like you said, the evidence is only as good as the system handling it.

Absolutely.

Let's set the baseline.

Forensic labs, they're not just places that run tests, you know.

They provide crucial scientific analysis.

They evaluate evidence, offer consultation, and really importantly,

provide expert testimony for the courts and law enforcement.

And forensic biology is a key part of that.

A core sub -discipline, yes.

It's amazing when you think about the scope.

It's definitely not just one person in a white coat looking at DNA.

A full -service forensic lab is.

Well, it's massive.

It really is.

You've got crime scene investigation teams out in the field, latent print experts looking at fingerprints, controlled substance analysis.

Drug identification, yeah.

Right.

Then post -mortem toxicology, finding out what substances were in a body, question document examination for forgeries, firearm, and tool mark analysis.

Comparing bullets and markings.

And explosives, fire debris, trace evidence like hairs and fibers.

It's a whole universe.

It is.

And within that huge operation, the forensic biologist's specific job often starts some biological evidence, maybe that blood stain or saliva or semen actually hits the lab bench.

And there's a very precise methodical process to turn that physical stuff into usable information.

Okay, let's unpack this.

So there's a standard procedure.

Step one sounds like maybe just basic sorting.

It's foundational, yeah.

Step one is identification.

And this is all about figuring out the class characteristics.

It's the first assessment.

What is this stuff?

Is that red spot actually Okay, if it is, is it human blood?

Ah, okay.

Class characteristics are these broad features that just place a sample into a big category with other similar materials.

It confirms the type of substance, but crucially, not yet whose it is.

Which is useful, I guess, for ruling things out quickly.

But it doesn't point a finger.

That big moment, legally speaking, must be step two.

Comparison.

Using what you call individual characteristics.

That's exactly right.

This is where we shift gears.

We go from just ruling people out to potentially linking a sample to a single, unique source.

Individual characteristics are those features,

unique patterns that, with a very high degree of certainty, can be traced back to one specific origin.

Like fingerprints.

Like fingerprints, or the most powerful tool we have today, DNA polymorphisms.

The scientist compares the characteristics of the evidence sample directly against a known reference sample, say, from a suspect or a victim.

The goal is to see if they could have come from the exact same person, or, just as importantly, to exclude someone if the profiles don't match.

And then the final step, step three, takes all that science and puts it into the legal context, reporting and testimony.

Precisely.

A detailed report gets written up.

It covers exactly what evidence was looked at, the methods used, the results found, and, critically, the conclusions drawn.

And for DNA, there's that statistical weight, right?

Absolutely crucial.

In DNA cases, you have to include statistical calculations that evaluate the strength of the conclusion.

How rare is this profile?

What's the probability of a random match?

Then the scientist often has to become an expert witness.

Taking the stand.

Yes.

Explaining all this highly technical stuff in plain language that, you know, attorneys, the judge, and especially the jury can understand.

It makes sense.

And thinking about those samples, they probably don't arrive in perfect condition very often.

I imagine biological evidence often needs help from other forensic specialists, too.

It's not just the DNA analyst working in a vacuum.

Oh, definitely not.

Think about how these fields interact.

You've got forensic pathology, for example.

The pathologist determines the cause and manner of death, natural, homicide, suicide, accident, or sometimes undetermined.

And time of death estimates.

Yes, estimating time of death through the autopsy.

That timeline is often critical for the investigation, and it affects the state of the biological samples the lab eventually gets.

And what about really old cases, or say, mass disasters, when things are badly decomposed?

That's where forensic anthropology often comes in.

They're experts in identifying and examining human skeletal remains.

They can potentially determine origin, sex, age, race, even signs of skeletal injury.

They play a huge role in identifying victims in mass fatality incidents.

Fascinating.

Then you also have forensic entomology.

That's the study of insects.

Bugs, right, on the body.

Exactly.

By looking at the types of insects present and their developmental stages, entomologists can estimate the post -mortem interval, or time since death.

Again, it helps build that timeline and contextualize the biological evidence.

Okay.

And one more forensic entomology, that's teeth.

Teeth, yes.

Dental records, x -rays, casts are incredibly useful for identifying victims, especially when other biological markers might be gone or too damaged.

They also analyze bite marks, comparing them to a suspect's dental structure, although bite mark analysis is, let's say, more debated scientifically these days.

Right, I've heard that.

But the point is, all these disciplines often work together, feeding information back and forth.

And this reliance on multiple fields, this constant push for better answers, leads us nicely into the history of forensic biology itself.

It's really a story of science constantly striving for more certainty, better individualization.

What's fascinating here is how it seems to have unfolded in distinct stages.

Right.

Like three major scientific shifts, each trying to improve on the last one's limitations.

That's a great way to put it.

Stage one, the very beginning, was antigen polymorphism.

This kicked off way back in 1900 when Karl Landsteiner discovered the human ABO blood groups.

A -B -O -A -B, stuff we learned in school.

Exactly.

And for decades, forensic serology, the study of bodily fluids, relied heavily on these blood groups.

But wait, type O blood is super common, right?

Mm -hmm.

And type A, finding type A blood at a scene.

Well, that doesn't exactly narrow it down much, does it?

It excludes some people, sure.

But the power to include someone seems weak.

You've hit the nail on the head.

The probability of a coincidental match to unrelated people just happening to share the same common blood type was quite high.

So scientifically, its value for inclusion was limited.

It was mainly useful for excluding suspects.

If the blood at the scene was type A and your suspect was type B, well, that's a clear exclusion.

Okay.

So useful, but not definitive.

Precisely.

And that scientific limitation, that need for better discrimination, pushed the field into stage two, protein polymorphism.

This started gaining traction around the mid -20th century.

Scientists began looking at variations, or polymorphisms, in serum proteins and certain enzymes found in red blood cells, like phosphoglucamutase or PGM.

So more markers.

They were trying to combine things like, if you're type A and have this specific protein variant and this enzyme type, it gets more specific.

That was exactly the idea.

By combining results from these protein and enzyme markers with the ABO blood groups, they could significantly lower the probability of a coincidental match compared to blood typing alone.

It increased the power of discrimination.

But still not quite pinpointing an individual.

Still not quite there.

It was better.

Definitely an improvement.

But forensic science was still searching for a method that could offer a much, much higher degree of certainty for individual identification, especially when dealing with tiny or mixed samples.

Which brings us finally to the revolution,

DNA polymorphism.

The revolution is right.

Now we're talking about the human nuclear genome.

Roughly three billion base pairs of DNA packed into 23 pairs of chromosomes.

The variations hidden within that genetic code between different people are the key.

Those differences are the polymorphisms.

Exactly.

DNA polymorphisms are simply variations in the DNA sequence from one person to the next.

For forensic work, we primarily focus on two main types of these variations.

Okay.

First, there's sequence polymorphism.

This is like a difference in a single letter, a single nucleotide at a specific spot in the DNA.

We call those SNPs, or single nucleotide polymorphisms.

All right.

SNPs.

But the real workhorse for modern forensics is the second type,

length polymorphism.

Here, the variation isn't in the sequence itself, but in the number of times a specific short segment of DNA is repeated one after another, like beads on a string.

These are called tandem repeats.

And that's where VNTRs and STRs come in, the terms you hear all the time.

Precisely.

VNTRs, which stands for Variable Number Tandem Repeats, sometimes called mini satellites, have repeat units that are relatively long, anyway from, say, nine up to maybe 80 or even hundreds of base pairs repeated over and over.

Okay.

Longer repeats.

But the star player today is the STR, or short tandem repeat.

These are also called microsatellites.

Their repeat units are much shorter, typically just two to six base pairs long, and that small size difference turns out to be incredibly important for practical lab work.

Why is the short size so critical?

We'll get to that in a moment when we talk PCR.

But first, just to make sure everyone's on the same page with the DNA basics, let's quickly lock down some core vocabulary you need to understand a DNA profile.

Okay, so the different versions of a DNA polymorphism at a specific location, or locus on a chromosome, are called alleles.

You inherit one allele from your mother and one from your father for each locus.

Right.

Two copies.

If those two alleles are identical, you are homozygous at that particular locus.

If the two alleles are different, you are heterozygous.

Homozygous, same, heterozygous, different.

Got it.

And the specific combination of alleles you have at a single genetic locus is called your genotype for that locus.

When you determine the genotypes across a whole panel of different STR loci, that collection of genotypes forms the individual's DNA profile.

The unique genetic fingerprint, essentially.

In essence, yes, although we prefer profile.

Now, applying this technology forensically has a really dramatic history.

The modern era really kicked off with Sir Alec Jeffries in the UK back in 1984.

He developed what was called multi -locus DNA profiling using those larger VNTRs we mentioned.

And that led to the first big case.

It did.

His work was instrumental in solving the first major crime using DNA evidence.

The identification of a double murderer named Colin Pitchfork.

It was huge.

Not only did it catch the real killer, but it also crucially exonerated an innocent man who had initially confessed under pressure.

Wow.

So it proved its power right away, both for conviction and exoneration.

Absolutely.

It showed that DNA could reveal far more individual variation than blood types or proteins ever could.

But analyzing VNTRs was still quite demanding, needing relatively large, good quality DNA samples.

So something else was needed to make it routine.

Exactly.

And the real game changer, the technology that truly democratized DNA analysis and brought it into labs everywhere, was the invention of the polymerase chain reaction, or PCR, by Carey Mullis.

PCR.

You hear that term constantly now.

And for good reason.

PCR is essentially a molecular copying machine for DNA.

It allows scientists to take a tiny, miniscule amount of DNA and amplify it, making millions or billions of copies of specific target regions.

So sensitivity went way up.

Traumatically.

Before PCR, you often needed a visible stain, maybe the size of a coin, to get enough DNA.

With PCR, analysis became possible from minute traces, a few cells left on a surface,

DNA from a single hair root, saliva on a cigarette butt.

Okay, now I see why the short size of STRs is so important.

Exactly.

That technological leap with PCR is precisely why STRs took over as the standard, replacing VNTRs and some earlier intermediate DNA markers, like one called HLA -DQA1.

Because they're easier to copy with PCR.

Yes.

Their small size, those short repeat units, makes them much easier to amplify reliably using PCR, even if the original DNA sample is degraded, broken down by time or environmental factors, or fragmented into smaller pieces, which is very common with crime scene evidence.

Makes sense.

Degraded samples must be a huge challenge.

A constant challenge.

Plus, STR loci are highly variable or polymorphic in the population.

When forensic scientists analyze a set of multiple STR loci, the standard in the US right now is a core set of 13 loci, soon expanding to 20 for the Combined DNA Index System, or CODIS.

CODIS.

That's the big national database, right?

That's the one.

When you look at all those loci together,

the probability of two unrelated individuals just happening to have the exact same combination of alleles, the same full STR profile, become astronomically small.

Statistically infinitesimal, really.

And CODIS lets labs compare profiles from crime scenes to profiles of convicted offenders, or other crime scenes.

Yes, it links cases.

It allows law enforcement agencies across the country to compare unknown crime scene DNA profiles against profiles from known offenders, and also against other unsolved case profiles.

It's been absolutely vital in solving countless cases, including many cold cases that were stagnant for years.

Incredible tool.

Are there other types of DNA testing used besides these standard STRs?

There are.

Sometimes the nuclear DNA, the STRs we've been talking about, is just too present and too low quantity.

In those situations, labs might turn to specialized techniques.

One is analyzing mitochondrial DNA, or MTDNA.

Mitochondria, the powerhouses of the cell.

They have their own DNA.

They do.

And importantly, you inherit your MTDNA exclusively from your mother.

Also, each cell contains hundreds or even thousands of copies of mitochondria, and thus MTDNA, compared to only two copies of nuclear DNA.

Ah.

So more target material available.

Exactly.

That makes MTDNA analysis very useful for really challenging samples, like old bones, decomposed remains, or hairs without roots, where nuclear DNA might be gone.

The limitation is that it's not as individualizing as STRs, because everyone in the same maternal line shares the same MTDNA sequence.

Okay.

And you mentioned Y -chromosomes earlier.

Right.

The other main specialized technique involves Y -chromosomal markers, specifically Y -STRs.

These are STRs located on the Y chromosome, which, as you know, pass down directly from father to son.

So only males have them.

Correct.

This makes Y -STR analysis extremely useful in certain situations.

Paternity testing, for instance.

Tracing paternal lineages.

But forensically, it's particularly valuable in sexual assault cases involving male perpetrators and female victims.

Why specifically there?

Because the victim's DNA is usually present in overwhelming abundance compared to the perpetrators.

Trying to pick out the male perpetrator's nuclear STR profile from that mixture can be tough.

But since females don't have a Y -chromosome, analyzing Y -STR specifically targets only the male DNA present, even if it's just a minor component of the mix.

That's clever.

Isolates the male contribution.

Yes, easily.

Okay.

Wow.

We've covered a lot.

From basic blood types to these incredibly specific DNA techniques.

It's a huge field.

Constantly evolving.

So what does this all mean?

I think the big takeaway for you, listening, is really understanding that journey.

The difference between early class identifying a stain as human blood and the power of modern individual characteristics, like matching a unique STR profile.

That leap from broad categories to near certain individualization is the whole story.

And remembering that PCR, the DNA copier, was the technological key that unlocked the analysis of tiny degraded samples, making STRs the powerful standard they are today.

The legal impact is just undeniable.

We went from methies that could only weakly include someone in large group, mostly useful for exclusion, to methods that provide incredible statistical power for individual identification.

A paradigm shift in forensic evidence.

Absolutely.

Now, here's a provocative thought to maybe mull over something mentioned in Lee's text.

We rely heavily on these STRs and VNTRs, these tandem repeats,

for identification, precisely because they're so variable between people.

But here's the thing.

Over 90 % of our entire human genome is made up of what's called intragenic non -coding sequences, DNA that doesn't code for proteins.

Right, the so -called junk DNA, although we know now much of it isn't junk at all.

Exactly.

And the biological functions of most of these sequences, including many of the specific tandem repeats we use every day in forensic labs, are still largely unknown.

We pick them because they vary, not because we necessarily know why they vary or what biological job they do, if any.

So consider this.

What if, tomorrow, research suddenly uncovered a crucial biological function for those highly variable STR markers we currently use purely as unique identifiers?

What if their variation suddenly had clear biological meaning, maybe related to disease susceptibility or some other trait?

How might that change things?

The ethical considerations, the legal interpretations?

It's fascinating that the bedrock of our most powerful identification tool still holds these biological mysteries.

It really makes you think about the layers of information potentially hidden in the evidence.

Thank you so much for tuning into this deep dive.

We really hope this journey through forensic biology was useful.

And we appreciate you choosing this summary focused on the chapter from Richard Lye's Forensic Biology.

A warm thank you from the whole Last Minute Lecture team.