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Genetics and Society: A Course for Educators
by Rob DeSalle, Ph.D., David Randle, Ph.D.
Dana Taylor
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DNA Fingerprinting, Individual Identification and Ancestry
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By Dr. Claudia Englbrecht (Guest Author)
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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 on
Chromosome 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.
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So why are microsatellites so useful for individual identifications? Why not use 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 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 of resolution
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.
Detecting
Microsatellite
Markers
This microsatellite
consists of tandem
repeats of the
dinucleotide CA. The
number of repeats in
such microsatellites
varies with the
individual (for
example: four repeats
in individual I and
eight repeats in
individual II). These
variations can be
detected by PCR
amplification with
primers that flank the
repeat region, since
this process yields
amplified DNA
products of different
lengths. ©NIH
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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 offer geneticists a large pool of
markers across which to establish a match.
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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
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based on those population databases. Without pre-existing data on these frequency
distributions, we could not assign a reliable probability value to any DNA fingerprint.
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.
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 Northern
Europe 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 widespread. 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." Science 298
(20 December 2002): 2381-2385.
Witherspoon, D.J., et al. "Genetic Similarities Within and Between Human Populations."
Genetics 176 (May 2007): 351-359.
Related Links
DNA on the Witness Stand »
Dr. Eric Lander, director of the Center for Genome Research at MIT, examines how genetics affects forensics
in civil and criminal court cases. As you scroll through the pages, you will learn about genetic individuality,
how DNA is used for forensic purposes, and the controversial proposal of a genetic database.
DNA Learning Center »
Explore these animations to learn about the procedure for making and reading a DNA fingerprint, and read
three cases that illustrate the process.
Confusions About Human Races »
Read this article from Harvard University professor Richard Lewontin on race and genomics.
Forensic Mathematics of DNA Matching »
Understand the calculation of DNA profile probabilities and what they mean.
Combined DNA Index System »
Get an overview on the laboratory procedures used by the Federal Bureau of Investigations for forensic DNA
evidence.
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