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What's In Your Blood?

This means less than 1% of your blood contains DNA!  

But did you know that only refers to nuclear DNA (nDNA), which is the DNA in your chromosomes, contained in the nucleus of the cell? 

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Deoxyribonucleic Acid

Is a sugar-phosphate backbone, which has base-pairs within, and acts as the blueprint for our bodies.

The human genome (complete set of genes) has over three billion base-pairs, with 99% being the same for all people. 

However, in that 1% difference, still lies a powerful level of individualization that has become useful to forensic investigation.

But did you know there is another kind of DNA?


There are small amounts of DNA in the mitochondria of a cell, which is referred to as mitochondrial DNA. However, this DNA is much smaller, as it has fewer base pairs than nuclear DNA (nDNA), and not all cells have mitochondria (e.g. red blood cells).  This mitochondrial DNA (mtDNA) is shared along the maternal side of a family line. So it cannot be used to identify a person specifically, but it may be used to help focus an investigation.  As mitochondria are larger numbers than nuclei, and prevalent in cells that do not have a nucleus, mitochondrial DNA may be present when nuclear DNA isn't!

Mitochondrial DNA

Mitochondrial DNA is both similar and different from nuclear DNA.  It is similar in that they are composed of the same types of base pairs(bp):

Cytosine-Guanine  &


However, it is different in both the number of base pairs, as nDNA has approximately 3.3 billion bp, while mtDNA only has 16,569 bp; and in structure, as mtDNA forms into a circular structure.

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Bloodstain Pattern Analysis

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To understand bloodstain patterns, you must appreciate the cohesion within blood.


When not acted upon by another force, blood forms into a sphere shape, due to strong cohesive forces.

As blood travels through the air, it is usually in a spherical shape (as pictured above), due to strong cohesive forces.  

Bloodstain Patterns

There are three basic types:

1. Gravity - created by force of gravity alone.  Commonly round, circular shaped stains. 

2. Spatter - created by more force than gravity alone. Amount of force affects stain size.


3. Transfer - created when blood moved from one location to another through direct contact.

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This teardrop shape is what many think blood looks like when it falls, as it is being acted upon by gravity.  But it is only in this shape when the upper tip is still connected to another surface.  

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Angle of Impact

Width/Length = Sin a (angle)

As such, if we have

W=3mm & L=6mm

Inverse Sin of W/L = a

Inv Sin 3/6 = angle

Inv Sin 0.5 =  30

Angle at which the blood drop came into contact with the surface, is 30 degrees




Good Sources of Nuclear DNA ?


The presence of saliva will often yield nuclear DNA, not because saliva itself has DNA, but because saliva is often mixed with cells that do

(e.g. epithelial cells, white blood cells) 


Blood is always a good source of nuclear DNA

(white blood cells) 


This was left in a crashed stolen car. The suspect was propelled to roof during the crash, his hat jammed into the sunroof, and he ran away leaving it behind. DNA from residual epithelial cells provided his identity!  

Don't Ya Just Love DNA!

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Detecting Unseen Blood

Finding blood at a scene, when it is not obvious or clearly visible, is not as easy as other human bodily fluids can be to find, as mentioned in the Forensic Light Source (FLS) area.  Blood does not fluoresce at any wavelength of light, and as such, it would only be a dark spot when any FLS wavelengths and filters are used.  Now, that constant dark spot under FLS may be an initial indication of latent blood, but there are many, many, other chemicals that do not fluoresce.  However, using chemical techniques, like LCV as seen in the Footwear impression area, is one way to find latent blood stains when they cannot be seen.  But, what happens when those stains are on a surface where a simple colour change, still won't make them very visible?

That's when it helps to turn out the lights!!


 Some chemicals, like Luminol or Blue Star, actually create light when they interact with blood. This reaction is called a chemiluminescence, and it can be amazingly useful way to find blood, where you wouldn't have looked before.  Here is an example where Blue Star was used:

The case is a violent one, and details are not needed, but in short people were hurt and the offenders got away in a vehicle.  Now a great investigation located a potential suspect vehicle, but there was a need to connect the vehicle, and by doing so, connect the owner of the vehicle to the occurrence.  And yes, there is far more than that needed to make that link a good one, but we are only focusing on establishing that first connection (so all you great investigators can be assured, that much, much, more was done). 

Now the event involved significant harm, and as such, a lot of blood at the scene.  A keen investigator thought, if there is blood at the scene, perhaps some of it got onto the footwear of the offenders (always a great place to look), and was subsequently left latent bloodstains in the vehicle.  Below, you see the front driver area of the vehicle, and there are no clear bloodstains.  

Note: the Canadian coin was put there by the investigator, and I'l explain that in a moment.  


The first step was to prepare the area for search.  What you see here is the area after the initial search, documentation, and photography was completed.  Then, the camera was put on a tripod, as it is essential for it to remain completely still in the photographs which will be taken!  And, lastly, a Canadian penny (1 cent coin) was placed in the area, where it was less likely for transfer to be present.  

Now the coin has two main purposes.  

  1. The first, is that the metal of the coin will provide a false positive for the test.  This will be an indication the solution is working, and help demonstrate what a positive result looks like in the dark.  

  2. The second, is the coin is an object of known size, which is a required reference in such a photograph, and you'll see why a ruler cannot be used in a moment.


Once the preparation and first photograph is taken, the next step is to turn off all the lights and switch your camera to a manual operation of the shutter, or an extended time exposure.  Now the settings can vary, but I've always found a 30 second exposure to work well.  Then, with the lights out, spray the solution onto the surface and as soon as you're done spraying take the next photo, of the same area, using the manual or timed exposure.  

It may not look like much, and your eyes may not see anything at all, but the camera can see record far more than the human eye can see! 

Below is what that second shot looked like in the case I mentioned.  You can see the positive reaction - blue glow - of the coin, and a few spots in the image where there is a very faint blue glow as well. However, it is virtually impossible to tell exactly where any other positive reactions may be from this image alone.   


Once the photograph is done, and at times it is best to take a few of these images successively, without adding anymore spray, as positive reactions may vary over the first few minutes.


However, once you are done, the next step involves overlaying the two images to help visualize where positive reactions may be, and where to sample for blood or DNA evidence specifically.    

In the image below is an example of exactly this, done using Photoshop. In the image you can see the area examined, as well as clearly see where the positive reactions took place - on the coin, on the brake pedal, and on the material in the upper right side of the image.  


Now, these positive results are nothing more than presumptive results! They are not, and cannot be taken as confirmatory, which means they allow for the presumption that latent blood stains may be present where the positive reactions occurred. But, there are a number of things, like the coin, that create false positives for this specific technique. 
In this case, samples, cuttings and swabs, where taken and submitted for DNA analysis.  As it has been in many cases before, this method of detection had no negative effects on subsequent DNA analysis, and is safe to use for this purpose.  The result was the method allowed the detection of latent blood stains, which were confirmed at the lab, and ultimately provided a DNA profile that demonstrated an important link for the case. 

It is important to remember, many of the techniques we use, will often be able to detect and see what we cannot.  

Just because you cannot see it, doesn't mean it isn't there!

"Every Contact Leaves A Trace" 

Never stop looking for those traces!