Todd Family Mystery

Name:

Period:

This teacher guide is provided to give sample answers to questions. Most of the questions are open-ended, so students may have correct answers that aren't included in this guide. Finally, although the experiment is set up to yield one correct answer, there are variations in data between students. As long as students examine their data carefully and can justify their answers based on their data, that's science! Data are always right and there isn't necessarily a 'right answer'.

The Scenario:

Last year Todd was an 8th grader at MLK Middle School in Salt Lake City, Utah. Todd is a pretty normal kid, but he had some things that always bugged him about his family, he did not look anything like his Mother and Father, nor his brother and sister. They all had brown hair and brown eyes, and even had dark complexion. Todd on the other hand had blue eyes, blonde hair and had a fair complexion. He also felt like he was very different personality wise, and his siblings always teased him that he had been switched at birth. In 8th grade Todd learned about genetics, and learned that there was a way to determine whether or not he was his parents’ child and, with his parents’ permission, we can help him confirm or deny his current biological parentage.

Some questions to get you thinking about today's lab:

What is DNA and what does it do?

deoxyribonucleic acid, makes you who you are, codes for proteins, etc.

What are chromosomes and how many copies do you have in each of your cells? Where did they come from?

Chromosomes are DNA packaged with proteins such that it will fit into the nucleus of the cell. You have 2 copies of each of the 22 autosomal (non-sex determining) chromosomes and either 2 X’s, if you are female, or an X and Y, if you are male. One entire set of 23 chromosomes (22 autosomal+1 sex chromosome) came from your mother and an entire other set from your father.

How is DNA passed on differently in the offspring of sexual reproduction compared to asexual reproduction?

One packet of chromosomes is provided by the mother (egg) the other packet of chromosomes is provided by the father (sperm). This process of sexual reproduction is virtually the same in all living things, including plants (offspring would be the seeds, sperm is in the pollen). If you have taught meiosis then introduce even more genetic diversity in the genes of Mom and Dad compared to the offspring. In humans a packet of chromosomes consist of 23 individual pieces. Compare this to asexual reproduction in which complete clones are created, most plants can clone but they also use sexual reproduction (refer to many cactus plants, lantana, or creosote to name only a few. These “new” plants are genetically identical to their “parent”.) Bacteria reproduce asexually with a process called binary fission, which is very similar to mitosis. Again the offspring are clones. Some animals reproduce clonally, a nice example around here at the whiptail lizards. Here in the Sonoran Desert there are no male whiptail lizards.

How much DNA do you share with each of your parents? How much DNA do you share with your siblings?

Parents: 50%

Siblings: Between 0% and 100% (baring in mind that all humans are 99.9% the same on a DNA level, these numbers are DNA that you received directly from your parents and that your siblings received directly from your parents.)

What are some characteristics or properties of DNA?

double-helix, double stranded, negatively charged, colorless, unique sequence in each individual

How can we take advantage of these properties to help us figure out if Todd is the son of his parents Frank and MaryAnn?

Since everyone has differences in their DNA, we can look at the regions that have measurable differences. These regions have been elucidated by the Human Genome Project, and the difference that are utilized for DNA fingerprinting are size differences for a specific part of a chromosome. First, we make a copy of the region with the size difference, then we separate out the pieces (since you have one piece from Mom and one piece from Dad you should have two different sizes, although you could have received the same sized piece from both) using Agarose Gel Electrophoresis. The DNA will migrate in an electric field, because it is charged (- charge therefore it will migrate toward the +), the agarose will offer the DNA resistance, bigger pieces have a harder time migrating than littler pieces.

Materials

• DNA from Frank, MaryAnn, Todd and his two siblings John and Anastasia
• agarose
• Tris-acetate/EDTA solution (TAE)
• micropipette/tips
• electrophoresis apparatus

Procedure:

1. Get your electrophoresis apparatus. Make sure the comb is placed closest to the BLACK ELECTRODE and that there are stoppers at both ends of the gel space. Why near the black electrode?

   DNA is negatively charged, so to move the DNA into the gel with electricity, the DNA needs to be loaded on the negative or black side, it will then move towards the red.

1. Pour hot agarose into the gel space until it reaches the top of the gel box. Let the agarose harden, which should take about 10 minutes. Don't touch/move your gel until it's hard. Why not? If the agarose moves while it's hardening, it will harden unevenly, making it more difficult for the DNA to move through evenly.
   
   Draw a picture of your gel and label which samples will be added to which wells before you add DNA.
   
   
   

2. Load your DNA SAMPLES into the wells, be sure to keep track of which samples you loaded in which lanes.

3. Now pour TAE solution over your gel CAREFULLY so that is it completely covered plus a little more. What do you think the TAE solution is for? TAE is like saltwater - it conducts electricity
   

4. Run that gel!! Plug the electrodes into your gel box (red to red, black to black), being careful not to bump your gel too much. Plug the power source into an outlet and set to 100 V. How can you tell your gel is running? It bubbles at the electrodes. This is a redox reaction, forming H2 gas at the black electrode and O2 gas at the positive electrode.

   If time permits, you will run the gel for about 30 minutes.
   
   
   

Analyzing Your DNA Data

Plug in your gel electrophoresis tray, and after 15 minutes draw a picture.

What can your data tell you about the relationship of Todd with Frank and Maryann?Is Todd the son of Frank and Maryann?

The data supports Todd being the child of Frank and Maryann. So why does he look so different. Most of Todd’s differences are due to recessive traits, if Frank and Maryann both were carriers of these recessive traits, therefore heterozygous, Todd could have received the recessive genes associated with the trait from each of his parents. This aspect of inheritance is luck of the draw. Brown hair, brown eye parents could have an offspring with blonde hair and blue eyes.

How much DNA do Todd and John share? Can you determine Todd and John’s relationship from the results of their two DNA samples? Please Explain.

Todd and John do not share any bands on this gel. We cannot determine Todd and John’s relationship from these two DNA samples. But other samples within their 46 chromosomes could elucidate that they are brothers.

What if you compared John with Frank and Maryann, does this help in determining the relationship of Todd and John? Please Explain.

The DNA evidence supports John’s relationship with Frank and Maryann as their son. Thus indirectly the evidence supports that John is Todd’s brother. Remember this is just one (two) small piece(s) of over 3(6) billion nucleotides. If John and Todd are Frank and Maryann’s kids, you would expect them to share DNA, probably at other sections of the chromosomes.

How much DNA do Todd and Anastasia share. Can you determine a relationship between Todd and Anastasia using these DNA samples?

Todd and Anastasia share both bands of DNA on this gel. Though this does not mean that they are identical twins. Just as with Todd and John, other sections of the chromosome may not be the same, or may be or they may share one (from mom or dad) but not the other.

What chance do siblings have sharing bands in this DNA fingerprinting technique?

Siblings have 50% chance of sharing any band, 25% chance of sharing BOTH bands, or NEITHER band.

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BIOTECH Project Department of Molecular and Cellular Biology The University of Arizona September 18, 2000 Nadja Anderson, PhD Last Modified March 1, 2002 http://biotech.biology.arizona.edu