Be a Crime Scene Investigator!


The term “DNA” is likely to be familiar to students from TV crime dramas and news reports. Use that familiarity to bring some real-world science into your classroom with a study of DNA and its use in forensic science.

The DNA molecule pictured above has two long strands joined together by bases of four kinds:

  • adenine (A)
  • thymine (T)
  • guanine (G)
  • cytosine (C)

You can list a person’s DNA “fingerprint”  by listing the bases: ATTGGCACGGTA and so on — for about 3 billion letters. Every human being has about 3 billion letters, and the chances that any one of us has exactly the same list of letters as anyone else are small. So small, in fact, that it probably never happens.

Some parts of the list are likely to be much the same in most people and some parts (the genes that determine eye color, for example) are likely to be very different. Forensic scientists check the DNA in some places that tend to show a lot of differences. The FBI’s DNA files, for example, have data from 13 different places in the DNA string.

The chances that you and I have the same string of letters in one place is about one in 10. That is, we would only need to have 10 people in the room to have a strong chance of having two people with matching DNA at that point. In a crime, there may easily be 10 different people who could have committed the crime, so DNA evidence from one place could only narrow things down. It couldn’t make an identification.

Combinatorics tells us that having the same string in two places in the DNA fingerprint would have the likelihood of 10 x 10. There would have to be 100 people in a room before we could expect to find two people who happen to match in two places.

A match in three places on the DNA string? The chances of that are 10 x 10 x 10: we’d need 1,000 people in a room before we’d have a strong chance of finding two people whose DNA matched in three places. Many crimes will have fewer than one thousand people who could have committed the crime, on the basis of access and opportunity.

The chances of matching in all thirteen places tested by the FBI are 10 to the 13th power: 1 in 10 trillion. There aren’t any rooms big enough to hold 10 trillion people, and the chances of a random match are infinitesimally small. This is why people usually feel pretty confident about DNA fingerprinting in court.

It’s different if there are family members involved, because family members usually have a much higher proportion of matching DNA. However, DNA fingerprinting has changed forensic science significantly from the days when actual fingerprints were the best method of identifying a criminal.

We like to start by reviewing information about DNA. Then go on to create a model of the DNA molecule. Use pipe cleaners and beads, candy, or a kit. Have students label the bases in a pattern of their choosing without discussing their patterns with other students.

Then check to see how many students happened to create matching strings. Here’s a simple way to do this:

  • Have students go to the four corners of the room, depending on whether their first base is A,C,T, or G.
  • Have each group subdivide, so that there is in each corner a group whose second base is A, a group whose second base is C, and so on.
  • Subdivide again for the third base, and so on.
  • At the end of the exercise, only those students with exact matches will still be standing together.

Now do the math.

Once students have grasped the concept, go on to further study. The resources listed include other areas of forensic science in addition to DNA fingerprinting, so you can extend the study to look at a number of other science concepts.


Online resources:

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