Forensics Notes
Fingerprint Analysis
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Skin
The skin is made up of three distinct layers.
The top layer is called the epidermis. (The word epidermis, and the name of the other
main skin layer, the dermis, both come from the name used by the ancient Greeks for the
skin, derma. From this we also get the word dermatologist, meaning a doctor who
specialises in skin problems.)
The epidermis is translucent. That is, it allows light to pass partially through it, rather
as frosted glass does. The epidermis does not contain any blood vessels but gets its
oxygen and nutrients from the deeper layers of the skin.
At the bottom of the epidermis is a very thin membrane, called the basement
membrane, which attaches the epidermis firmly, though not rigidly, to the layer below.
The second layer lies deeper and is called the dermis. It contains blood vessels, nerves,
hair roots and sweat glands.
Below the dermis lies a layer of fat, the subcutaneous fat. The depth of this layer
differs from one person to another. It contains larger blood vessels and nerves, and is
made up of clumps of fat-filled cells called adipose cells.
The subcutaneous fat lies on the muscles and bones, to which the whole skin structure
is attached by connective tissues. The attachment is quite loose, so the skin can move
fairly freely. If the subcutaneous tissues fill up with too much fat the areas of attachment
become more obvious and the skin cannot move as easily -this is what gives rise to the
notorious cellulite.
Fingerprint Formation
The uniqueness of fingerprints (epidermal ridges) has been recognized for over two
thousand years. They have been studied scientifically for more than two hundred years.
Yet, in spite of the accumulation of a wealth of empirical and experimental knowledge,
no widely accepted explanation for the development of epidermal ridges on fingers,
palms and soles has yet emerged. Informed by an extensive literature study we suggest
that fingerprint patterns are created as the result of a buckling instability in the basal cell
layer of the fetal epidermis. Analysis of the well-known von Karman equations informs
us that the buckling direction is perpendicular to the direction of greatest stress in the
basal layer. We propose that this stress is induced by resistance of furrows and creases to
the differential growth of the basal layer and regression of the volar pads during the time
of ridge formation. These theories have been tested by computer experiments. The results
are in close harmony with observations. Specifically, they are consistent with the wellknown observation that the pattern type depends on the geometry of the fingertip surface
when fingerprint patterns are formed.
Interpreting DNA Chromatograms
This section is a brief overview on how to interpret your chromatogram and how to recognize
what is good, reliable sequence data and what is not. In general, the least reliable areas of
sequence are at the very beginning and towards the end, and we’ll show you what it looks
like. With a little practice, you’ll be able to look at your trace files and immediately get a sense
of how well your sample sequenced. Below is an example of how a well-prepared, accurately
quantitated DNA sample might sequence. Check in our Troubleshooting Your Data section for
some examples of what less than optimal data might look like.
Sequence data at the beginning:
This is an example of a chromatogram from our pGEM control included on every run. Note that
the first ten peaks or so are somewhat jumbled, rounded and not too well resolved. There’s
also a little bit of unincorporated dye, but not so much that it becomes too interfering. The
peak height is also somewhat smaller at the beginning. Notice that the two A’s at bp 19-20 are
not well separated. All of this is normal, but reinforces the suggestion to place your primer
about 40-50 base pairs away from your sequence of interest so as to avoid these issues. Also,
keep in mind that your primer will NOT be visible in your sequence data - the primer is not
labeled, but the first nucleotide that extends off from it is. So, theoretically, the first
nucleotide in your sequence should be the first nucleotide after your primer. However, as you
can see from the example above, even with our ultrapure and accurate control the earliest
data is not very reliable so you shouldn’t expect to be able to confidently analyze your
sequence from base#1.
Sequence data towards the middle:
This is where the good stuff is. Notice how well defined and sharp the peaks are, with nice
even spacing between them. Peak height is also significantly higher than the earliest
fragments. There is little or no background interference present at the peak baselines. This
sequence would be considered 100% accurate, from a visual perspective.
Sequence towards the end of the sequence:
Now you can see where the resolution is beginning to deteriorate. The peaks are no longer
sharp but have become broad and more rounded in shape, especially where you have more
than one of the same nucleotide in a row. The peak height is also reduced. While this region
has still been basecalled with 100% accuracy (this control, in fact, was 99% accurate out to
base #920), it can be difficult to manually interpret, and it would definitely not be advisable to
design a primer from a region like this. Sequence at the end like this might be useful when
looking for sequence motifs or trying to determine exon-intron boundaries, but probably
nothing much more specific than that.
Glass Analysis
If the recovered fragments match the reference sample both in RI and chemical composition it is
necessary then to discover how rare or otherwise the glass might be. To do this the scientist may
consult a computer database containing the combined results of the RI measurements and the
chemical analysts results for each and every reference glass sample examined by his lab. This will
show how many times a particular type of glass has been encountered, but this is not necessarily a
reliable indication of how common it might be. This is because most glass samples submitted to
forensic laboratories are from broken windows, although research has shown that glass found on
clothing by chance is more likely to have originated from a container (such as a glass or a bottle).
Aside from this, there tend to be local pockets of specific types of glass, e.g. from buildings all built
at the same time. All this means that information held on the database might be skewed and should
be treated with caution.
Aside from databases, the forensic scientist will also need to consider for example, the number and
distribution of the glass fragments he has recovered and whether this fits in with the alleged
circumstances.
Entomology
Flies (Order Diptera)
Flies are often first on the scene. They prefer a moist corpse for the maggots to feed on,
as such a corpse is easier for them to chew. The most important families are:
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Blowflies - Family Calliphoridae
Fleshflies - Family Sarcophagidae
House Flies - Family Muscidae
Cheese Flies - Family Piophilidae
Coffin Flies - Family Phoridae
Lesser Corpse Flies - Family Sphaeroceridae
Lesser House Flies - Family Fanniidae
Black scavenger flies - Sepsidae
Sun Flies- Heleomyzidae
Black soldier fly- Stratiomyidae
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Beetles (Order Coleoptera)
Beetles are generally found on the corpse when it is more decomposed. In drier
conditions, the beetles can be replaced by moth flies (Psychodidae).
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Rove Beetles - Family Staphylinidae - are elongate beetles with small elytra (wing
covers) and large jaws. Like other beetles inhabiting carrion, they have fast larval
development with only three larval stages. Creophilus species are common predators of
carrion, and since they are large, are a very visible component of the fauna of corpses.
Some adult Staphylinidae are early visitors to a corpse, feeding on larvae of all species of
fly, including the later predatory fly larvae. They lay their eggs in the corpse, and the
emerging larvae are also predators. Some species have a long development time in the
egg, and are common only during the later stages of decomposition. Staphylinids can also
tear open the pupal cases of flies, to sustain themselves at a corpse for long periods.
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Hister Beetles - Family Histeridae. Adult histerids are usually shiny beetles (black or
metallic-green) which have an introverted head. The carrion-feeding species only become
active at night when they enter the maggot-infested part of the corpse to capture and
devour their maggot prey. During daylight they hide under the corpse unless it is
sufficiently decayed to enable them to hide inside it. They have fast larval development
with only two larval stages. Among the first beetles to arrive at a corpse are Histeridae of
the genus Saprinus. Saprinus adults feed on both the larvae and pupae of blowflies,
although some have a preference for fresh pupae. The adults lay their eggs in the corpse,
inhabiting it in the later stages of decay.
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Carrion Beetles - Family Silphidae
Ham Beetles - Family Cleridae
Carcass Beetles - Family Trogidae
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Skin/Hide Beetles - Family Dermestidae. Hide beetles are important in the final stages
of decomposition of a carcass. The adults and larvae, which are hairy, feed on the dried
skin, tendons and bone left by fly larvae. Hide beetles are the only beetle with the
enzymes necessary for breaking down keratin, a protein component of hair.
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Scarab Beetles - Family Scarabaeidae
Sap beetles - Family Nitidulidae
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Mites (Class Acari)
Many mites feed on a corpse. Macrocheles mites are common in the early stages of
decomposition, while Tyroglyphidae and Oribatidae mites such as Rostrozetes feed on
dry skin in the later stages of decomposition.
Nicrophorus beetles often carry on their bodies the mite Poecilochirus which feeds on fly
eggs. If they arrive at the corpse before any fly eggs hatch into maggots, the first eggs are
eaten and maggot development is delayed. This may lead to incorrect PMI estimate.
Nicrophorus beetles find the ammonia excretions of blowfly maggots toxic, and the
Poecilochirusmites, by keeping the maggot population low, allow Nicrophorus to occupy
the corpse.
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Moths (Order Lepidoptera)
Clothes-moths - Family Tineidae - feed on mammalian hair during their larval stages and
may forage on any hair that remains. They are amongst the final animals contributing to
the decomposition of a corpse.
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Wasps, ants, and bees (Order Hymenoptera)
The insects in this group, order Hymenoptera, are not necessarily necrophagous. While
some feed on the body, some are also predatory, and eat the insects feeding on the body.
Bees and wasps have been seen feeding on the body during the early stages. This may
cause problems for murder cases in which larval flies are used to estimate the post
mortem interval since eggs and larvae on the body may have been consumed prior to the
arrival on scene of investigators.
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Wasps – (particularly) Family Vespidae
Ants – Family Formicidae
Bees - Superfamily Apoidea
Blood Spatter Analysis
As mentioned earlier a blood droplet in freefall has the shape of a sphere. Should the
droplet strike a surface and a well-formed stain is produced, an analyst can determine the
angle at which this droplet struck the surface. This is based on the relationship between
the length of the major axis, minor axis, and the angle of impact.
A well-formed stain is in the shape of an ellipse. Dr. Victor Balthazard, and later Dr.
Herbert Leon MacDonell, realized the relationship of the length-width ratio of the ellipse
was the function of the sine of the impact angle. Accurately measuring the stain will
easily result in the calculation the impact angle.
Fig. 2 Upward moving bloodstain showing proper ellipse placement.
Fig. 3 Angles of Impact
Angles of Impact
Because of the three dimensional aspect of trajectories there are three angles of impact,
, , and . The easiest angle to calculate is gamma ( ). Gamma is simply the angle of
the bloodstain path measured from the true vertical (plumb) of the surface (see figure 2
showing plumb line and extended angle from stain.) The next angle that can be quite
easily calculated is alpha ( ). Alpha is the impact angle of the bloodstain path moving
out from the surface (see figure 2 with alpha at the top by the stain.) The third angle to be
calculated is beta ( ). Beta is the angle of the bloodstain path pivoting about the vertical
(z) axis (see figure 2 with beta extended to the floor). All three angles are related through
trigonometry through the equation quoted below.
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Calculating the α angle
= length of ellipse (major axis)
= width of ellipse (minor axis)
= angle of impact
The relationship between these variables is:
Therefore:
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Relationship between angles α, β, and γ
Accurately measuring the stain and calculating the angle of impact requires due diligence
of the analyst. In the past analysts have used a variety of instruments. Methods currently
used include:
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Viewing loop with an embedded scale in 0.2 mm increments or better that is placed
over the stain. The analyst then uses a scientific calculator or spreadsheet to complete the
angle calculations.
 Bloodstain Pattern Analysis (BPA) software that superimposes an ellipse over a scaled
close-up image of an individual bloodstain. The programs then automatically calculates
the angles of impact.
Using software produces a very accurate result that is measurable and reproducible.
Seeds and Pollen