*Review sheet- Chapter 7 Inman*
*Interpretation of **DNA** typing results: *
Different Factors
*Mixtures*- This occurs when there is more than one human individual has
contributed biological material to an evidentiary sample, multiple
DNAprofiles may be detected.
--sexual assault cases- usually represented by sperm cells and non-sperm
cells. The none sperm cells in sexual assault evidence originate from the
vaginal epithelium of a female victim termed *epithelial cells*
Extractions- If the Differential extraction is successful an analyst can
then interpret both types of DNA if it is *unsuccessful* this may lead to
patterns with more than two alleles per locus.
Y-STRs are also providing to be useful in resolving sexual assault mixtures
that for one reason or another have been recalcitrant to standard autosomal
analysis.
*Non-sexual assault mixtures-*
For example, this type of mixture may be found in a crime scene where many
people are bleeding. *RFLP *analysis remains the standard for resolving
mixtures. This is because so many alleles exist in the population at each
locus that individuals rarely share alleles.
Y-STRS can also play a role in figuring out mixtures as well. But there is
no set rules to exactly how non-sexual assault mixtures should be analyzed.
*Degradation*
This occurs when DNA is subjected to any environmental outside the body, it
is not as stable as when it is inside the body. Environmental conditions
that lead to these changed include *time, temperature, humidity, light and
chemicals.*
PCR based test are generally better suited for the analysis of smaller
DNAfragment size than RFLP.
WHY? Because the regions chosen for STR analysis are not only very small
(<500bp), but they are very close to each other in size. Because of this it
is not only more likey that a sample subjected to extreme conditions of
heat, humidity or chemicals exposure will yield a result. But the possibily
of losing an allele at a heterozygous locus due to degradation is
practically eliminated.
*Extraneous Substances *
Chemical inhibition of enzymes- Due to the necessity of enzymes in PCR and
RFLP any agent that interferes with an enzymatic function will potentially
inhibit their ability to act on DNA.
Example: Denim Dye & Biological substances (in particular the red-colored
heme in blood)
Some ways to overcome inhibition- adding more Taq polymerase or adding
bovine serum albumin (BSA). BSA seems to bind and disable inhibitors.
Non-human DNA- all living things in the world contain DNA. A possible
source that may be found in a crime scene are; Bacteria and Fungi. The
occurrence of Non- human DNA is not troublesome, per se, because the DNA
test used for forensic purposes are designed to be human (or higher
primates) specific. However, the presence of microorganisms that produce
DNA-destroying enzymes contributes to the degradation of human DNA.
Slot Blot quantification test is made exactly to analyze how much
human/higher primate DNA is present.
*RFLP*
Because the RFLP loci used in forensic DNA testing are so highly variable
(two bands per locus) a deviation from this "norm" might be due to *genetic
variation, artifacts found on the **DNA** or generated by analytical
conditions.*
More than two banded pattern- analyst must take into account the possibility
of a mixture or whether the profile represents a single individual and extra
bands are artifacts
Partial digestion by the restriction enzyme- this occurs when the
restriction sites along a strand of DNA are chemically blocked
Bands running off the bottom of the gel- only occurs where a particular loci
is of very small base pair. Running of the gel occurs in the gel
electrophoresis when the smaller bp allele runs of the gel rendering an
allele a homozygous rather than a heterozygous.
Summary of RFLP Interpretational issues
The immense diversity of these loci between people is what makes them so
attractive for forensic work also presents analytical challenges and renders
them susceptible to environmental induce alterations. These challenges
emphasize the need for training and education for analyst.
*PCR*
Mixed samples, degradation and contamination are all concerns when apply a
PCR analyst to an evidence sample.
Amplification- also plays an important part that may result in
complications when reading the results of PCR based analysis. *Stochastic
fluctuation *occurs when only a portion of a DNA extract is removed to
another tube for amplification.
*Perferential amplification-* refers to the amplification of one allele to a
much greater degree than another one in the same reaction. This would result
in a heterozygote appearing to be a homozygote. Also called *Allelic dropout
*