We have all seen forensic scientists in TV shows, but how do they really work? What is the science behind their work? The course aims to explain the scientific principles and techniques behind the work of forensic scientists and will be illustrated with numerous case studies from Singapore and around the world. Some questions which we will attempt to address include: How did forensics come about? What is the role of forensics in police work? Can these methods be used in non-criminal areas? Blood. What is it? How can traces of blood be found and used in evidence? Is DNA chemistry really so powerful? What happens (biologically and chemically) if someone tries to poison me? What happens if I try to poison myself? How can we tell how long someone has been dead? What if they have been dead for a really long time? Can a little piece of a carpet fluff, or a single hair, convict someone? Was Emperor Napoleon murdered by the perfidious British, or killed by his wallpaper? *For Nanyang Technological University (NTU) students, please be noted that this course will no longer be eligible for credit transfer.
Week 3B - 1 Blood
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Del curso dictado por Nanyang Technological University, Singapore
Introduction to Forensic Science
381 calificaciones
De la lección
Time of Death; Blood
Conoce a los instructores
Roderick BatesAssociate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences
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The topic of this lecture is Blood and the forensic uses of blood evidence.
This topic is sometimes called forensic serology.
In a violent crime, there is likely to be blood spilled.
There may be blood on the victim,
there may be blood around the crime scene,
and there may be blood on the suspect.
If it's fresh blood, then it's fairly obvious that it is blood, but if it's old,
dried blood, it's not always possible to tell
just by looking at it that it is blood.
So we need to have some tests to find out if
these stains on the suspect or the crime scene actually are blood.
And in some cases, for instance, if the crime scene
has been cleaned up, there may be no visible signs of blood.
So we need a test to show up these signs of blood.
Once you've found blood and you've determined that it
is blood, then the question is, is it human blood?
And if it is human blood, whose blood is it?
Now, in a violent crime, where there's a lot
of blood scattered around the crime scene, another question arises -
can we use the pattern of how the
blood is spattered to reconstruct the events that occurred?
So these are some of the questions we'll address in this lecture.
Now, what is blood?
It's a very, very complex substance. It makes up about 8%
of our body weight, so there's a lot of it about.
Now blood is, as we know, a liquid.
But it's only just a liquid, so the fluid portion of blood, which
is the blood plasma, is just over half of the blood by weight.
And this consists of water and various other biochemicals
such as proteins, nutrients, and also waste products from our metabolism.
So, blood acts as a transport system to move all these substances around the body.
Now, blood also contains blood cells and
this makes up the remainder of the material,
and there are different cells for doing different things.
The red blood cells, their principal job is oxygen transport.
It's to transport oxygen from the lungs down to the cells where it's needed.
There's also the white blood cells.
These are responsible for our immune response.
And then there's also platelets, and these are responsible for the clotting response.
Let's look closely at the red blood cells.
They're not very big.
They're about 7.8 microns in diameter,
so you can get about 5 million in a microlitre.
They have this particular concave shape, as shown in the picture.
And one of the odd things about red blood cells that makes
them different from all our other cells is that they contain no nucleus.
That means that the red blood cells, as
they contain no nucleus, do not contain DNA.
So if we're getting DNA evidence from blood, then that
DNA must come from the other cells that are present.
As we've mentioned, the principal function is transport of oxygen
by binding of oxygen to the hemoglobin molecules that are present.
On the surface of the red blood cells,
there are some special proteins which are called antigens,
and it's these antigens that are
responsible for the phenomenon of blood typing.
Now there's many, many different kinds of these antigens.
In fact, there's well over 100, if you can
include some of the rarer antigens as well.
But a few of the antigens are much more common than the others,
and it's these antigens that give us the A-B-O system of blood typing.
You know very well that if you have a blood
transfusion, you must be given blood of the right type.
Now, in the very early days when
blood transfusion started and nobody understood blood type,
some people would get a blood transfusion and be fine, and some people would
get a blood transfusion and they would be very ill or they would die.
And this confusion was only sorted out by Karl
Landsteiner in the early part of the twentieth century,
and he won the Nobel prize for medicine
for developing the A-B-O system of blood typing.
Now, how does blood type work?
Well, if you have type A blood, it means that on
the surface of your red cells, you have the A antigen.
If you have type B blood, then on the surface
of your red blood cells, you have the B antigen.
If you're a type AB, then on the surface of the cells
you have both sets of antigens, both the A and the B.
And if like me you're a type O, then on the surface
of the red cells, you have neither the A nor the B antigens.
Now, the percentage of people who have these
different blood types varies from country to country.
Here in Singapore, the O is the most common, it's at about 40%,
and the AB is the least common, down at about 5%.
Now, what does blood type mean?
The only thing blood type means is what
kind of antigens you have on your red cell surfaces.
It doesn't mean anything else.
In some countries however, it's the belief, it's commonly
thought that your blood type also dictates your personality.
So this movie for instance here asks, is your blood type the key to true love?
Well, the answer to that is no.
And here is a former minister of the Japanese government,
who got into a lot of trouble and had to resign
because of some insensitive comments that he made,
and his excuse was that he was a type B.
In reality, blood type doesn't mean anything like this.
Now to go with the antigens, we have the antibodies.
The antibodies are big protein molecules which
make up a large component of our blood,
and they are produced as part of our immune system.
The purpose of an antibody is to bind to
any objects in the blood that they recognise as alien.
Okay, so anything in the blood that shouldn't
be there should be bound up by an antibody.
So if you look at the structure of the antibody, it's Y-shaped,
and you can see two hands sticking out,
and that the, those hands on the end, these are the antigen binding sites.
The important thing about the antibodies
is that the interactions are extremely specific.
The antibody will only bind to its target and not to anything else.
So the blood serum can contain antibodies that correspond
to the surface antigens of the red blood cells.
So there's the anti-A antibodies and the anti-B antibodies.
So if you're blood type A, then your blood will contain the anti-B antibodies,
because if you're type A, then the B antigens would be alien to your blood.
If you're type B then of course, you'll
contain the anti-A antibodies, because A would be alien.
So you have the antibodies which are the opposite to your blood type.
So if you are AB, where you have both
sets of antigens, then you don't have either kind of antibody.
If you are type O, where you have neither kind of
antigen, then your blood will contain both sets of antibodies.
A type A person will have the anti-B antibodies in the blood.
These antibodies cannot bind to the antigens on the type A red cells.
Now remember that the antibody has two hands, it has two binding sites,
which means that the antibodies, the anti-B antibodies, and the
B red cells will clump together in this complex network.
This is called agglutination,
and this will result in serious medical problems.
As I mentioned before, there are many different antigens, and the
A-B-O typing system is based on just the A and the B antigens.
Of the less common or less often used
antigens, the one that's more studied is the D.
And this is why when you talk about your blood type, you don't
just say A, AB, B, O, you also add positive or negative.
So if you have a positive blood type, then it means you have the D antigen.
If you have a negative blood type, then you do not have the D antigen.
So on average, about 85% of people
are rhesus positive and 15% are rhesus negative.
So if we recalculate the percentage of people
with the different blood groups using this information,
then O positive becomes the most common, and AB negative becomes
the least common, at about 1% of the population here in Singapore.
Now, let's think about blood type in terms of forensic science.
One of the objectives of forensic science is to
take evidence from the crime scene and individualise it.
That is, to associate it with a single person.
And if you look at the percentages here, you can see that
blood type, or blood group, is really not very individualised at all.
When you talk about the common types, like O positive or B positive,
that's a lot of people.
And even when we go to the least common blood type, the
AB negative, we are still talking about 50,000 people in this country.
So blood type is really not very individualised at all.
Well, to address this problem, studies were carried out on the less common
antigens to try and get towards individualisation of blood type,
and a lot of work was done on studying these antigens.
But then everything changed.
In 1990 or so, DNA technology was introduced,
and DNA technology means that you can take
a blood sample and you can individualise it.
And therefore, all this research into the forensic uses of
these minor antigens stopped, because it had been superseded by DNA.
That doesn't mean that blood type is not important,
it just changes the way that we use blood type.
So blood type cannot be used to prove guilt
because it's not individualised.
Blood type can be used to establish innocence.
Suppose at the crime scene, the blood found there
is type A and your suspect is type B.
Then that suspect ceases to be a suspect and you have established his innocence.
But if that suspect was type A, it doesn't prove that he was guilty.
It just means you have to go on and look for further evidence.
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