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Forensic Biology (continued)
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4. In criminal paternity or mass disaster victim identification,
determination of the haplotype of a missing individual
may be conducted by typing a male relative.
5. In sexual assaults, the time-consuming and sometimes
inefficient differential extraction procedure for the separa-
tion of sperm and nonsperm fractions may be bypassed.
6. Y chromosome typing may provide increased statistical
discrimination in mixture or kinship analysis cases in which
the discrimination obtained from autosomal markers is
insufficient for identification purposes.
Interpretation
Once amplified and typed, the results need to be interpreted.
Forensic biologists must have a clear understanding of the
molecular methods used, with in-depth knowledge of the basis
of typing (that is, the cell biology, technology, and genetics of
the loci), the specific loci and amplification parameters (deter-
mined both externally and internally in developmental and
internal validation studies), the empirically derived limitations
of the system, the protocols and quality control measures
implemented in their own laboratories, the instrument valida-
tions, the analytical software used in determining the DNA
profiles, and the statistical and population genetics databases
and software used.
In single-source samples, the interpretation requires setting
a threshold of detection. That is, alleles that are at or above
a certain fluorescent threshold may be designated. In highly
degraded DNA samples, the interpretation requires more
analysis, as higher-molecular-weight loci generally degrade
more rapidly than lower-molecular-weight loci. In this case, the
alleles are expected to appear in decreasing amounts as the
size of the alleles increases (see Fig. 3). In mixtures, the inter-
pretation requires considering all combinations of alleles that
are present, allele ratios within and among loci, and the sample
type and condition.
Fig. 3: Results of degrading DNA by mechanical disruption. (a) Agarose gel electropherogram of intact DNA and degraded DNA, electrophoresed in lanes 2 and 3 through a 1% agarose gel,
stained with ethidium bromide, and then photographed with UV light. Molecular-weight ladders of Lambda Hind III were placed on either side of the samples in lanes 1 and 4. The sample in
lane 3 has been highly degraded with no detectable high-molecular-weight band as is apparent in lane 2. (b) Capillary electropherogram of degraded DNA stored at room temperature ver-
sus frozen. The decrease in PCR product amount as base pair size increases, as shown in the top panel, is the expected result for a degraded sample. The result demonstrates the importance
of proper (frozen) sample storage.
