Modern DNA analysis
Critical biological samples were taken from the victims, from suspects
and from the collected evidence. The appeals attorney petitioned
the courts to have DNA testing performed on this case evidence.
The state and the appeals attorneys agreed to examine the following
evidence.
Samples from the victims and their clothes.
- Routine
fingernail scraping from each victim that may or not have tissue from
the perpetrator (each hand samples saved separately, three victims for
six evidence samples).
- Ligatures from each of the three victims
- Tissue, skin or hair samples obtained from the ligatures of each victim.
- Swabs taken from each victim.
- Cuttings from blue jeans.
- Cuttings from blue pants.
Anomalous hairs taken from the victims.
- Two Caucasian hairs removed from Moore.
- Dyed hair removed from Moore.
- Hair found on Byers' body
- Hair on lower body, Byers
- Hair, perineum, Byers
- Two dark Caucasian hairs removed from Branch.
- Although
all hairs from the ligatures were requested as mentioned above,
specific hairs were also requested from the bindings Moore and Byers
ligatures.
Anomalous hairs from clothing and fabrics in contact with the children and the crime scene.
- Negroid hair removed from white sheet 1.
- Hair from blue pants.
- Hair from Scout cap.
- Hair on tree stump (crime scene)
And, finally, other items.
- Bag of clothing, area of homicide.
- Hair from two knives from Richard Cummings.
- Hair from knife from Jason Crosby/Richard Appling.
- Wooden stick, crime scene.
Biological specimens were also obtained from 44 other suspects and family members for matches or exclusion. These
samples can be tested by the modern DNA techniques of STR
fingerprinting and mtDNA. The results can then compared to known
samples from the children, the convicted and others to determine the
source.
STR Fingerprinting
Along with the meaningful DNA that encodes for
physical traits, we have a lot of "junk DNA," strings of repeating
patterns and filler material. STR stands for short terminal
repeating sections, and this area of DNA represents a pattern, 2 to 7
letters long that repeats itself many times. How many times
varies between people. Any given STR sequence is about as good as
HLA DQalpha technique for determining
relatedness. Fortunately, there are such sequences on each
chromosome, each with its own length. By testing nine of these
sites on different chromosomes you get a one in a billion unique
signature. Nine sites as standards are used by the military for
paternity matters. Thirteen sites are commonly used for forensic
tests and for the CODIS database.
To perform
STR analysis, you divide up a sample for separate PCR reactions.
Each yields a product of a specific length of DNA, the length
determined by the number of repeats within the sequence. These
are then run on a gel to determine the exact length of each
sequence. Again a specific stepladder-like pattern of DNA pieces
with different lengths can be compared to a source sample.
Presented below are six STR loci, using the analogies of sentences.
The various sequences "yada_yada," "etc_etc_etc" are present in
differing amount of copies among individuals.
Suspect A
Site_one_yada_yada_yada_yada_yada_yada_site_one_end (51 characters)
Site_two_oy_oy_oy_oy_site_two_end (33 characters)
Site_three_huh_huh_huh_huh_huh_huh_huh_huh_site_three_end (49 characters)
Site_four_ooga_ooga_ooga_ooga_ooga_ooga_site_four_end (53 characters)
Site_five_whatsit_whatsit_whatsit_whatsit_whatsit_whatsit_site_five_end (65 characters)
Site_six_booga_booga_booga_booga_booga_booga_site_six_end (57 characters)
Suspect B
Site_one_yada_yada_yada_yada_site_one_end (41 characters)
Site_two_oy_oy_oy_oy_oy_oy_oy_oy_site_two_end (45 characters)
Site_three_huh_huh_huh_huh_huh_huh_huh_huh_site_three_end (57 characters)
Site_four_ooga_ooga_ooga_ooga_ooga_ooga_site_four_end (53 characters)
Site_five_whatsit_site_five_end (30 characters)
Site_six_booga_booga_booga_booga_site_six_end (45 characters)
The first PCR reaction makes copies of the DNA beginning with the
sequence "site_one" through "site_one_end" and the other reactions
proceed with "site_#" through "site_#_end." The DNA products are
separated on a gel. The results appear something like this.
STR products separated on a gel extending the above analogy using sentence fragments.
In this case the sample from the crime scene
matches suspect A.
Matching a
sample to a known suspect is not the only way to find the source of the
DNA evidence. The United States has a database of gene sequences known
as CODIS. If the suspect is arrested in another crime that requires
his or her DNA to be examined it can be matched. Or, if an evidence
from another crime scene is taken, the DNA can be sequenced and entered
into the database thus linking two crimes.
With high sensitivity and an excellent ability to discriminate among
all but identical twins, is STR fingerprinting the perfect method?
With 13 sites needing to be amplified contamination of multiple
runs would be necessary for the results to lead to a false
identification. However, contamination of a small portion of the
runs could result in a missed identification.
A one in a
trillion chance of matching doesn't mean a trillion to one odds that a
person is guilty. Samples are still vulnerable to human errors
and faults include mislabelling and evidence falsification. In
regards to the latter, there are examples where suspects have gone out
of their way to make sure a wrong sample is provided.
 Cross-section of a cell. The purple bodies are mitochondria.
Mitochondrial DNA
Mitochondria are small bodies that exist inside
of cells and provide the cell's energy. They have their own DNA
and divide. The process of making spermatazoa eliminates
mitochondria from the sperm cells, therefore all of our mitochondrial genes are
inherited unchanged from the egg cells of our mothers. Mitochondrial DNA
is small by comparison to nuclear DNA - only 16,000 "letters."
In the case of mtDNA, two highly variable regions are sequenced.
This results in the entire area of DNA "spelled out."
The forensic scientist then compares the suspect sample to the evidence
sample. The analysis can be a bit complex, but when the sequences
are identical they are determined to be a match. The FBI does not provide
official statistics on how unlikely it is that two unrelated people
would have the same type of mtDNA. The odds are probably greater
than a thousand to one. If people are related through the same
mother (or maternal grandmother and so on through the maternal line)
they will have the same mtDNA.
There are several advantages and disadvantages to the use of
mtDNA. Some samples can not be analyzed using STR or
other nuclear DNA fingerprinting methods because they only have mtDNA.
These may include hair shafts, bones and teeth. Also, since cells
have multiple mitochondria, this method is better able to find DNA in
minute or highly degraded samples. The disadvantages are the
lack of resolution among related individuals and the smaller number of
mtDNA samples entered into the SORIS database. While some
think of mtDNA as the method used when only hair samples are available,
the root of hairs have nuclear DNA and STR fingerprinting is also an
option.
MtDNA inheritance across three generations.
Individuals with the same colored circles share the same mtDNA.
Although many have the same mtDNA, even maternal half-siblings,
there are still eight mtDNA types represented among these 21 individuals.
[Continued in Part Three. Findings from the West Memphis Case.] |
