dna fingerprinting inividual identification and ancestry help

7/29/2019 DNA Fingerprinting Inividual Identification and Ancestry Help

1/3

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


2/3

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.


3/3

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.

dna fingerprinting inividual identification and ancestry help

Documents