Analyzing Genetic Variation

A Mix-Up at the Hospital

About This Activity

Class periods required: 1/2-1

Introduction

The DNA sequence contains much more variety than is seen at the phenotypic level of proteins or outward traits. This variety can be detected by restriction fragment length polymorphism (RFLP) analysis, by polymerase chain reaction (PCR) with variable tandem repeat regions, or even by the sequencing of short regions of DNA. Unlike a blood type, a person's DNA sequence is as individual as his or her fingerprint (with the exception of identical siblings, who share DNA sequence but have different fingerprints).

Although DNA typing is clearly more accurate in determining family relationships, blood tests can often give a simple "yes," "maybe," or "no" answer. Blood typing is less expensive to conduct than DNA typing, is a routine procedure of all major hospital laboratories, and is fast. Therefore, blood typing is usually the first test used in trying to establish family connection or to determine whether two blood samples came from the same person.

Objective

1. Explain how to use DNA profiles to determine whether a couple are the parents of a particular child;
2. Give a clear description of what to look for in comparing the profiles.

Materials

• Photocopies of the Student Activity pages if students do not have manuals
• For younger students, the posters of human chromosomes (parents and offspring) from the meiosis game.

Preparation

Procedure

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To analyze the DNA profiles, students should carefully compare the babies' profiles to the profiles from each couple. For example, students can start with one baby's profile and compare it to the profile for the first couple. First, they should focus on one member of the couple and compare the baby's profile to that person's (for example, the woman's). Each band in the baby's profile should be checked to see if it matches a band in the first woman's profile. Any bands that match should be marked lightly in pencil. If no bands match, the woman cannot be the baby's mother. If some bands match, the student should compare the remaining bands to the man's DNA profile. Every remaining band in the baby's profile will match a band in the man's DNA profile if that man is the father. If some but not all bands match, then the couple are not the parents of that child. The student should go on to the next couple and compare the baby's profile to it in the same manner.

If you want to use this activity with younger students who have not completed the Southern hybridization, PCR, and DNA typing lessons, you will have to decide how much you want to tell them about how the DNA profiles are generated. They will need to be familiar with the activity of restriction enzymes and with gel electrophoresis of restriction fragments to understand the activity.

Start the lesson by showing them the posters of parental and offspring chromosomes from the meiosis game. Remind them that everyone is different because their chromosomes are different and that half a person's chromosomes come from the mother and half come from the father. Ask them if they think that by comparing chromosomes, they could tell whether a baby belonged to a particular couple. You might even ask them what they would look for.

Tell your class that scientists look at small regions of chromosomes when they want to tell whether people are related. These regions are different in nearly everyone.

The following explanation is intended only as an example of how you might prepare a younger class for the activity.

Ask your class a series of questions about what effect a highly variable DNA sequence would have on restriction sites. Continue asking questions until students come to the idea of RFLPs (the term is not necessary). It is not necessary to explain blotting and probing. Keep using your posters of chromosomes to generate examples. Make up a short sequence for a region of both of dad's chromosomes and both of mom's chromosomes, and then let the offspring have one of each. Let the class figure out the restriction fragments from mom, dad, and the offspring. (The chromosome 19s of Bob and Mary in the Student Activity Analyzing Genetic Variation: DNA Typing would be perfect examples.)

After the class has figured out the restriction patterns, draw these patterns in a gel outline on the blackboard. Make sure the class sees that each band in the child's profile has a counterpart in the parents' patterns but that not all of the parents' bands are present in the child's profile. Now have the class make up a different pattern for a second child of Bob and Mary (the second offspring could inherit the other chromosome 19s from the example). Ask the students what they think a profile from an unrelated child might look like (the important thing is that the bands would not match Bob's and Mary's).

When you feel your class is ready, give a short introduction, telling the class that they will be using DNA profiles to decide which baby belongs to which couple. (You could ask them if they have ever heard of babies being mixed up at a hospital.) Hand out the activity sheets. If you think it is necessary, go through one comparison with your class, following the procedure outlined previously. As your students work on the puzzle, circulate through the classroom and observe their work to make sure they understand what they are doing.

Answer

Optional Follow-Ups

• Invite a speaker from a local hospital to talk about identification methods such as blood typing and DNA typing.
• Have students make up their own DNA typing puzzles.

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