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7/29/2019 DNA Fingerprinting Inividual Identification and Ancestry Help
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DNA Fingerprinting, Individual Identification and Ancestry Help
By Dr. Claudia Englbrecht (Guest Author)
Genetic fingerprinting, a molecular technology that allows individuals to be
identified based on their DNA, has become central to forensics, paternity
testing, conservation biology, evolutionary biology and ancestry research. It
would be hard to find a television episode of CSI that doesn't mention this
technology. But what is a genetic or DNA fingerprint? How accurate are they?
How much information about an individual does a genetic fingerprint actually
hold?
In the 1980s, well before the Genomic Revolution, researchers discovered
that our genomes contain large amounts of so called "repetitive DNA." In
thousands of locations, basic short motifs like GA or CAG are repeated in our
genetic code, to read something like GAGAGAGAGA or CAGCAGCAGCAG a
"genome stutter". The key to the diagnostic power of a genetic fingerprint lies
in understanding these repetitive elements, which are called microsatellites
or short tandem repeats (STRs).
In principle microsatellites behave like genes. Let's say that Location X onChromosome 1 harbors a microsatellite. We have two copies of Chromosome
1 and therefore we also have two copies of the microsatellite. Both copies
have the same core sequence, e.g. GA, but they can have different numbers
of repeats. One copy could have five repeat units, GAGAGAGAGA, and the
other copy could have eight repeats, GAGAGAGAGAGAGAGA. Unlike genes
that can vary in sequence, the repetitive elements can (but do not have to)
vary in the number of repeat units.
So why are microsatellites so useful for individual identifications? Why notuse genes that code for blood type or hair color?
Microsatellites have another important characteristic: they are extremely
variable. For some of them we can find up to 20 or more different length
variants in the human population; the core repeat unit may be replicated
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four, six, eight, ten times, or more. Most single copy genes in our genome are
much less variable and only come in a limited number of versions (alleles);
for example, the gene that determines your blood type exists as just three
different alleles: A, B and O. Each individual can harbor only a maximum of
two different length variants per microsatellite analogous to two alleles in
single gene copies. You can see that microsatellites have a higher power ofresolution than, for instance, the alleles for blood groups. The probability that
you have the same two length versions at a microsatellite locus as your
neighbor is lower than the probability that you share the same blood type,
because there are more microsatellite variations in the population.
Compare this to a lottery in which you pick two numbers out of a pool. If the
pool contains only three numbers, the likelihood of other people picking the
same two numbers is high. If the pool contains 20 numbers, it's far less likely
that somebody else will get the same number as you. Microsatellites offergeneticists a large pool of markers across which to establish a match.
However, examining just one microsatellite locus is not sufficient to make
DNA profiling unambiguous - especially in a death penalty case. There is still
a chance that you and your neighbor have the same two microsatellite-length
variations.
A forensic DNA profile is reliable because several different microsatellite loci
are studied at any given time, and because geneticists have compiled
databases of the frequencies of microsatellite versions across human
populations. Consider the same lottery, but imagine that you had ten
opportunities to pick two 2 numbers out of a pool of 20. The probability that
you and another person would choose the exact same numbers all 10 times
is extremely low. But you cannot go to court and say, "The probability is
extremely low, so the suspect has to be convicted." Forensic scientists must
present solid statistics that yield exact probabilities. Their statistics are based
on those population databases. Without pre-existing data on these frequency
distributions, we could not assign a reliable probability value to any DNAfingerprint. The probability of any two individuals having an exact match in
the microsatellite combinations used in forensics is typically something like
one in several billion. This is why courts of law allow DNA profiling to identify
people. Conversely, if a suspect's DNA does not match that found at a crime
scene, it is immediately clear that he or she did not do it.
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One criticism of the use of DNA fingerprints is the data underlying the
probability calculations. Is the database really representative of the
population? If not, the calculated probability values will be biased. However,
the biases are known and are taken into consideration.
A typical genetic fingerprint, which looks on average at ten different
microsatellites, does not reveal anything about your personality, your mental
capabilities, your ethnicity or possible predispositions to disease. However,
exhaustive studies on human populations from all over the world have shown
that if we look at a far larger number of specific microsatellites (several
hundred), we can roughly determine that individual's ancestry. How is that
possible if humans are genetically so similar, you might ask? This is because
the worldwide distribution of certain alleles is uneven. For example length
variant 1 could be very common in Central Africa, but less so in Asia and
Northern Europe, while length variant 2 could be more abundant in NorthernEurope than anywhere else, and so forth. The uneven distribution is due to
the evolutionary history of modern humans, who migrated from Africa
through the Middle East and on across the other continents. Nowadays, we
have large sets of microsatellite data (and of other genetically informative
elements). We also know a lot about the genetic variability of populations
around the world. Most alleles are wide-spread. Only very few alleles occur in
specific regions and are genuinely rare. The map of human genetic variation
also shows that there are no abrupt changes in frequencies, but rather
continuous, gradual changes between populations and continents.
DNA tests are a very powerful tool inside and outside the courtroom.
Inevitably, they involve serious ethical and legal issues. Is DNA fingerprinting
really the perfect evidence? Are the results always accurate? Who should be
in these databases? Only convicts, people under arrest or the entire
population? Would you want your DNA profile in a large database or would
you prefer to keep your genetic profile private?
Related Resources
Rosenberg, Noah A., et al. "Genetic Structure of Human Populations." Science298 (20 December 2002): 2381-2385.
Witherspoon, D.J., et al. "Genetic Similarities Within and Between Human
Populations." Genetics 176 (May 2007): 351-359.
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