Download Genetics and DNA Fingerprinting

DNA Fingerprinting
Short Tandem Repeats Each individual has unique DNA sequences making up about 1% of their genome. Some of the
unique sequences are called short tandem repeats. These are sets of repeated, short sequences of bases that are
tandemly arranged (one after the other) along the length of a chromosome. For example, TTTTC or CGG could be a
tandem repeat. The sequence of nucleotides could then be repeated anywhere from 5 to 50 times. The number of
short tandem repeats differs from individual to individual.
TTTTCTTTTCTTTTCTTTTC…
or
CGGCGGCGGCGGCGGCGG…
Process PCR is used to amplify a region of a chromosome known to have tandem repeats. Several regions are actually
used. The size of PCR generated fragments differs among individuals because the number of tandem repeats also differs
among individuals. An individual with only 5 repeats will generate short fragments while an individual with up to 50
repeats will generate long fragments. The fragments are separated by gel electrophoresis and then visualized using
nucleic acid hybridization (Southern blotting).
Evaluating DNA Fingerprints Each individual’s DNA is loaded in one well so that the bands in the lane corresponding to
that well are the “fingerprint” for that individual. In the example below, Bob has 8 tandem repeats while Joe only has 2.
When their DNA is amplified using PCR, the fragments from Bob will be longer than those from Joe. When the DNA is
run on a gel and visualized with methylene blue, ethidium bromide, or nucleic acid hybridization, Bob’s band of longer
fragments will migrate slower and thus be closer to the top of the gel than Joe’s.
Bob
Joe
The banding pattern is different for each individual. If only one area of tandem repeats was tested then there
would be a lot of similar DNA fingerprints. However several regions are tested. If a least three regions are tested there
would be a 1 in 1X1018 chance of two people having the same banding pattern (except for identical twins). 1X1018 is far
more than the number of people on Earth
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Crime Scene
Evaluate the following DNA fingerprint from a crime scene and answer the questions. (Actual DNA fingerprint analysis
contains more bands. This fingerprint is simplified for ease of use and learning.)
Questions
1. Which suspect’s blood was found at the scene of the crime? (Compare the suspect’s band pattern with blood
found at the scene of the crime.)
2. Is the victim’s blood on the knife?
3. Who else’s blood is on the knife?
4. What can be determined by the bands from blood on the knife?
5. Which suspect is guilty?
6. Can DNA fingerprinting be used to prove guilt? Explain.
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Genetics and DNA Fingerprinting
Tandem repeats are inherited in a Mendelian fashion. Just as a gene can have different alleles, the tandem
repeats can have different numbers of times they are repeated. Each diploid cell will have two tandem repeat patterns.
Just as with genetic crosses the offspring inherit one tandem repeat pattern from their father and one from their
mother.
For example if a dad had a fingerprint consisting of bands of 800 and 400 base pairs, then you could expect his
offspring to have either the 800bp band (see child 1 and 3 in the figure) or the 400 bp band (see child 2 and 4). The
offspring would not get both bands because they will only inherit one chromosome from the father (remember meiosis).
If the mother had bands of 900 and 200 base pairs, then again the offspring would have one or the other band. The
offspring would have a band from the father and from the mother. Comparing offspring with parents can demonstrate
relatedness. The pictured gel is just a possible example, however typical DNA fingerprints have many more bands and
are much more complex, but they still show this basic inheritance pattern.
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Genetics and DNA Fingerprinting
Determine what possible tandem repeat patterns the offspring of this Dad and Mom could have. Dad has “repeat Q” on
one chromosome and “repeat R” on the other chromosome. The DNA fragments from each chromosome could be
generated using restriction enzymes as show with the scissors or PCR. In either case many copies of the same two
fragments for each parent would be generated which would migrate at a specific speed through an agarose gel resulting
in a band each (see depiction of the gel electrophoreses below)
Dad
repeat Q
repeat R
Mom
repeat S
repeat T
Treating each tandem repeat of the dad and mom like alleles in a genetic cross do the Punnett square to show the
possible combinations of tandem repeats the offspring could have. Child 1 is done as an example—they receive Q from
dad and S from mom.
Mom
S
Dad
Q
Mom
T
QS
Child 1
Child 2
Child 3
Child 4
Dad
R
On the gel below label the dad’s and the mom’s DNA bands (Q, R, S, and T) in the text boxes to the left. Use the
depicted length shown above. The longest fragments migrate the slowest, so you can label the band that migrates the
slowest with the letter designating the longest fragment.
Based on the Punnett square, place the red bands on the DNA gel where the bands would be for each possible offspring.
Each square on the Punnett square is labeled child 1-4. The possible offspring in square 1 becomes child 1 on the gel.
Child 1 has been completed as an example.
Bands
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