Page 20 - An Identity Crisis

alleles and are not included. A compilation of these STR loci compared.
STR comparisons from as may as 16 different,
independent STR loci make up an individual’s Siblings, however, will not share the same
genetic profile. statistically random chance of receiving their
alleles, as they can only receive the alleles their
Genetic profiles are created for both unknown parents possess. Therefore, it may require testing
samples from a crime scene and those that are of many additional STR loci to separate siblings
obtained directly from a person who may be a using DNA typing. Further, identical twins share
suspect. Those collected from people, whether identical DNA and cannot be separated.
a suspect, victim, or any other individual, are
referred to as “knowns” and are used to compare DNA analysis has revolutionized forensic
known profiles against unknown evidence profiles. identification. Each of the technologies outlined
An individual is excluded as a suspect when their here have illustrated a step forward in this rapidly
profile does not match the questioned sample’s evolving discipline, however, as technology and
profile exactly, and they are included only when the laboratory technique grow together further
fragment sizes are identical at every STR locus in innovations in the science of determining unique
the DNA. identity are assured.

STR DNA typing is an examination of the length of
certain repetitions in a genome. In order to weigh
the importance of each match, the final step in
DNA typing is a statistical analysis. By sampling a
large number of people in a particular population,
researchers have determined allelic frequencies ADDITIONAL READING
for the STR loci used in forensic DNA typing. By Butler, J.M. (2005) Forensic DNA Typing:
multiplying the population frequencies of the alleles Biology, Technology, and Genetics of STR
present in a genotype, the probability of another Markers (2nd Ed). Elsevier Academic Press,
person in a population having the same alleles at New York.
each loci can be determined. As the number of
loci in the analysis increases, the probability of a National Research Council (1996)
random sample matching decreases geometrically. Evaluation of Forensic DNA Evidence.
Thus, the probability is the result of both the National Academies Press, Washington D.C.
rareness of the individual alleles, the distinctiveness
of their combination, and the number of different

20 THE MYSTERY OF LYLE AND LOUISE