Lesson Explainer: Applications of DNA Fingerprinting Biology • First Year of Secondary School

In this explainer, we will learn how to outline the basic principles of DNA fingerprinting and recall some applications of it.

If you are familiar with modern detective dramas, you have probably heard of DNA profiling, often called DNA fingerprinting. By matching the DNA of a suspect to a sample found at a crime scene, investigators are able to determine someone’s probable involvement with a serious crime. If unknown human remains are uncovered, samples taken from the body can be used to produce a DNA fingerprint and help identify the deceased.

This technology is also used in paternity testing, helping individuals research their family tree, and in medical research. DNA profiles can be used to identify if an individual has a section of DNA that is associated with a particular genetic disorder. DNA profiling techniques can also be used to identify organisms and determine their evolutionary relationships by comparing how similar their DNA fingerprints are. They can even be used to study how a body reacts to a transplanted organ or tissue!

In order to further understand the applications of DNA fingerprinting technology, we will first investigate the principles behind the technique. Then, we will look at how they are applied to forensic investigations and paternity testing.

The credit for developing DNA profiling technology is often given to Sir Alec Jeffreys, a British geneticist who successfully used the technique to solve a high-profile murder case in the 1980s. However, the technique was also developed independently by an American scientist named Jeffrey Glassberg who patented it around the same time.

The principles of DNA profiling have remained largely unchanged since that time, while the actual techniques used have progressed with technological advancement.

DNA is the genetic material found in the nucleus of each of our body’s cells. It possesses a “code” in the sequence of nucleotide bases: A, T, C, and G. The human genome, or our entire genetic sequence, contains billions of these bases. However, only about 1- of them make up what we call “genes” in a human. Genes are segments of DNA that are transcribed into RNA, which is usually translated into proteins. These proteins determine our traits and characteristics. DNA is passed down from parent to offspring. A child will inherit half their genetic information from their mother and half from their father. This is how traits get passed from one generation to the next.

While all humans have about of their genetic code in common, there are also parts of our genetic code that are individually unique. Except for the case of identical twins, each human has a specific genetic sequence that is theirs alone!

The most recognizable differences between individuals are in the long, noncoding regions of DNA in each of our chromosomes. When we say that a region of DNA is noncoding, we mean that this DNA does not code for a protein. The majority of the human genome is noncoding, which we can see in the graph in Figure 1.

Both Alec Jeffreys and Jeffrey Glassberg independently discovered that the repetitive, highly variable segments of noncoding DNA are a reliable way to identify individuals. They used this knowledge to develop a technique to visualize these differences.

They would start by taking a sample of a person’s DNA. Then they would use an enzyme that breaks that DNA at the locations of a certain sequence of nucleotides. This results in many fragments of DNA that have different sizes. The nucleotide sequence the enzyme recognizes appears in different places in the DNA sequences of different individuals. This means that the same enzyme used on DNA samples from two different individuals would result in a different number of fragments with different sizes, which is illustrated in Figure 2.

However, our genome is billions of bases long, and it is mostly the same among members of our species. How can we just visualize the parts that are unique?

After breaking up long DNA molecules with enzymes, scientists use gel electrophoresis to separate the DNA fragments based on their size. In this process, DNA samples are loaded into wells at the end of a gel, then an electric current is passed through the gel. This causes the DNA fragments to move, and they are separated according to size—smaller fragments move further than larger ones.

Then, they use a special type of dye that marks the DNA. You can see the result of a DNA profile created by gel electrophoresis in the photograph below!

The pattern of stripes that results is what we call a person’s DNA fingerprint. Each person has a unique DNA fingerprint since their repetitive DNA segments are unique and the sizes of the fragments are unique as well!

It can be difficult to study complex DNA fingerprints like the one shown above, but Figure 5 shows a simplified drawing that allows us to understand exactly what is being shown. In Figure 4, we have taken the DNA segments shown in Figure 2 and outlined what they might look like if they were separated by gel electrophoresis.

Now that we are familiar with what a DNA fingerprint is and how it is made, let’s examine how they are used.

DNA fingerprinting today is commonly used in forensic science as a method of determining a person’s involvement in a crime. Newer genetic technologies mean that we can get a DNA fingerprint from a very small sample of a person’s DNA. Scientists take a sample of DNA from a crime scene. This could be obtained from the cells in a blood splatter, a hair, or even tiny amounts of skin left behind when someone touched a surface! They amplify, or make many copies of, the DNA and then make a genetic fingerprint using the technique we described earlier. Then, the scientists do the same thing using samples of suspects’ DNA. Finally, they compare the DNA fingerprints of the suspects to the ones gathered from the crime scene to determine a match.

We know that each person’s genetic fingerprint is unique since the sizes of the repetitive DNA fragments will be unique. The scientists match up the different stripes in the sample from the crime scene to the various suspects, as seen in Figure 5. When they find a suspect who has a similar pattern in their DNA fingerprint, they can conclude that this person was present at the crime scene. It is important to note that DNA fingerprinting alone is not enough to convict a person of a crime, but it proves that they were (or were not) at a certain place.

In Figure 6, we see the analysis that leads to the conclusion that suspect 3 has a DNA fingerprint that matches the sample from the crime scene. We can see that out of the samples taken from the three suspects, suspect 3 has bands that closely match the sample from the crime scene.

How To: Comparing DNA Fingerprints

A DNA fingerprint shows fragments of repetitive DNA separated by size. Larger fragments are near the top of the image and smaller fragments are closer to the bottom.

When scientists use DNA fingerprints, they are often looking for a “match.” This means that a test sample has fragments of the same size as a reference sample.

Sometimes, a partial match provides the information we need. In this case, a test sample has some fragments that are the same size as a reference sample and some that are not.

In order to compare the sizes of fragments in a reference sample to those in a test sample, it is useful to draw lines or boxes that contain the bands or stripes that represent the DNA fragments in the reference sample and to extend these lines or boxes horizontally to intersect the locations of the one or more test samples.

In the example above, column 1 shows our reference sample. Column 2 is not a match to the reference since none, or an insignificant portion, of their DNA fragments are the same size as those in the test sample. Column 3 would represent a partial match since about half of the fragments match with those in the test sample. And column 4 is an example of an exact match since all of the fragments are the same sizes as those in the test sample.

Example 3: Using DNA Fingerprints to Determine Which Suspect was Likely Present at the Scene of a Crime

A DNA fingerprint was constructed from samples of hair found at a crime scene. DNA fingerprints from samples taken from 4 suspects are also constructed.

Based on the information in the diagram provided, state which suspect’s DNA was found at the crime scene.

Answer

The DNA fingerprint of each person is unique. It is a visual representation generated by breaking the DNA apart into fragments of various sizes and then using a special dye to indicate the fragments that include repetitive sequences. Using this technique, scientists are able to profile individuals based on just the DNA that most clearly sets them apart from each other. In order to use a DNA fingerprint to determine the likelihood of the involvement of a suspect with a crime, first a DNA fingerprint is made from a sample acquired at the crime scene, in this case, a hair sample, and then DNA fingerprints are made from DNA samples taken from the suspects. Then, the suspects DNA fingerprints are compared to that of the crime scene sample in order to identify a match.

In the diagram below, the darker bands of the DNA taken from the crime scene have been highlighted with green boxes. Any dark bands in the suspects’ DNA that match the crime scene DNA have also been highlighted with a green box.

By simply looking inside of these boxes, we can determine whether the sample from a particular suspect has DNA fragments that match those from the sample found at the crime scene. If a suspect’s DNA fingerprint matches in this way, we can conclude that they were present at the crime scene and left the sample of DNA that the investigators found there.

We can see that suspect 4 has no fragments that match that of the DNA found at the crime scene. Suspects 1 and 3 have two matches highlighted. But suspect 2 is showing a high proportion of matches.

Based on the analysis of these DNA fingerprints and their comparison to the DNA fingerprint from the crime scene sample, suspect 2’s DNA was found at the crime scene.

Another common use of DNA fingerprinting is in determining biological parentage. Since we inherit half of our genetic material from our biological mothers and half from our biological fathers, half of our DNA fingerprint is likely to match that of our mothers and the other half is likely to match that of our fathers.

In order to determine paternity using genetic fingerprinting, scientists acquire 3 samples of DNA: one from the mother, one from the child, and one from the potential father. They follow the steps to generate a DNA fingerprint from each person. They then match the segments the baby inherited from their mother. Then, they determine whether the remaining segments were likely inherited from the potential father. The DNA fingerprints of a mother, father, and child can be seen in Figure 7.

If there is a question of paternity, this same technique can be used to determine who is most likely the father of a child from several potential candidates. In a method similar to that of forensic DNA fingerprinting, the DNA fingerprint of the child can be compared with several potential fathers to determine who is a match.

Example 4: Using DNA Fingerprints to Determine Which Male Is Likely the Father of a Child

The paternity of a child is in dispute. A DNA fingerprint was constructed for the child and their mother. DNA fingerprints from samples taken from 4 possible fathers are also constructed.

Based on the information in the diagram provided, state which male is most likely to be the child’s father.

Answer

A DNA fingerprint is a visual representation generated by breaking the DNA apart into fragments of various sizes and then using a special dye to indicate the fragments that include repetitive sequences. The DNA fingerprint of each person is unique, except for the case of identical twins. DNA fingerprinting is commonly used to accurately determine the parentage of an individual. Children inherit half of their genetic material from their father and half from their mother. This means that half of the DNA fingerprint of a child is likely to overlap with that of their mother and half is likely to overlap with that of their father. In order to use a DNA fingerprint to determine the likelihood of a male being the father of a child, first, a DNA fingerprint is made from a sample acquired from the mother, the child, and all the potential fathers. The child’s DNA fingerprint is analyzed in comparison with their mother. Then the portions of the child’s DNA fingerprint that do not have a match in the mother’s DNA fingerprint are compared against each of the potential fathers to see if their pattern is a match for those segments.

The dark red squares are drawn to surround the darker bands, or stripes, in the child’s DNA fingerprint and extended to intersect with the DNA fragments of the same size in the mother’s DNA fingerprint. This shows us which of the fragments in the child’s DNA fingerprint were inherited from their mother. We would expect the remaining fragments to have been inherited from the child’s father. The blue boxes are drawn around these remaining fragments in the child’s DNA fingerprint and extended to intersect with each of the potential fathers. By simply looking inside of these boxes, we can determine whether the sample from a potential father has DNA fragments that match those belonging to the child and that were not inherited from the mother. If a male’s DNA fingerprint matches in this way, we can conclude that they are most likely the father of the child.

Based on this type of analysis, we can conclude that male 3 is most likely the father of the child.

Popular crime dramas have idealized the use of genetic fingerprinting, which is still a relatively young technology. Issues with sample contamination, degradation over time, and variations in the interpretation of results mean that DNA fingerprinting is often not the faultless evidence we expect it to be. However, advances are being made all the time, and DNA fingerprinting has already been used to solve many crimes and exonerate many innocent people.

Let’s summarize what we have learned about DNA fingerprinting from this explainer.