Activity 1: Calculating the probability of genotypes

1. Hand out the Double the Muscle: Genotype and Probability worksheets, coins, and calculators to each group of 2-3 students. Discuss the procedures and expectations of the lab. (Use books as a back stop for coin flips if needed.)
2. Read page 1, Introduction to the Double Muscle Trait with students to explain that they will be using their coins to represent the random chance of a parent contributing 1 of 2 alleles (represented by heads and tails of the coin).
3. Use a coin to illustrate the following as the steps of the activity.
   • Step 1: Calculate gametes for the bull.
   • Step 2: Calculate gametes for the cow.
   • Step 3: Compare the bull and cow results.
   • Step 4: Predict offspring genotypes from a mating of a heterozygous bull and cow.
   • Step 5: Calculate the probability of each offspring type.
   • Step 6: Reflect on the coin tossing results.
4. As students begin step 1, draw blank tables on the board for students to fill in their coin toss data from steps 1 and 2 on the worksheet. 
5. Assist students as needed to calculate the percentage of head and tail flips for the entire class. It is recommended that you work through an example calculation.
6. Have students share and compare their findings from steps 1 and 2 in step 3. Help students connect the idea that their equal chance of getting heads or tails in a coin toss is the same probability that happens when gametes are formed and receive one of two alleles. In step 1, the heterozygous bull is just as likely to give the dominant allele as the recessive allele. The same is true for the heterozygous cow in step 2.
7. In step 4, students will determine the four possible combinations when 1 allele is contributed from each parent and understand that these combinations represent the potential genotypes of the offspring (DD, Dd, and dd).
8. Discuss whether Dd and dD are phenotypically distinct from each other. They are not distinct from each other and the probability of getting heads and tails in any order would be found by adding the 2 separate probabilities (¼ and ¼). Phenotype and genotype probabilities can be illustrated using a sample Punnett square on slide 6.
9. Before step 5, assign students to work in pairs (if they aren’t already doing so) and provide each group with two different coins. This will make it easier for students to keep track of coin flips simulating the bull gametes and the cow gametes.
10. For step 5, encourage students to predict how often the DD, Dd, dD, and dd genotypes will be flipped (as a percentage) and decide how many coin tosses should be done as a class to adequately test their prediction.
11. Pairs of students will conduct as many coin tosses as needed to feel confident in the genotype probabilities. Have students contribute their data to a class data table on the board.
12. Have students calculate the total number of coin tosses completed for the whole class.
13. Assist students as needed in calculating the percent coin tosses in Step 5, #5.
14. Assuming students conducted a reasonable number of trials, all allele combinations (or genotypes) should have approximately a ¼ chance of being tossed.
15. For evaluation, have students complete step 6 on their own. The answer to Step 6, #1 is ¼ and uses the product rule. The product rule states that the probability of independent events occurring together is equal to the product of the probabilities of these events occurring separately. For example, if you tossed 2 different coins simultaneously, the probability of both turning up heads is as follows:      

Activity 2: Using a Pedigree

1. Explain to students that you are going to further explore Mendelian genetics by applying their knowledge to a pedigree.
2. Discuss how a knowledge of inheritance and probabilities can help make predictions that, when used in conjunction with pedigrees, can help breeders select for desirable traits or select against undesirable traits.
3. Distribute the Double the Muscle: Using Pedigrees worksheet to each student and read page 1, Introduction to Pedigrees which tells about the beef rancher, Walter, and his cattle herd.
4. Following the reading, help students recall their prior knowledge of pedigrees. Using the pedigree from the student worksheet, discuss the basics of how pedigrees work. Remind students that:
   • Circles represent females
   • Squares represent males
   • Shading  indicates that an animal or plant has a specific trait.
   • The horizontal lines that connect circles to squares indicate a mating (or a marriage in humans).
   • The vertical lines coming from a “marriage” line indicates that the individuals had offspring.
   • Multiple offspring are shown with a vertical line connecting to a horizontal line where several vertical lines can be drawn and connected to offspring.
5. Discuss the purpose of pedigrees. In genetics, pedigrees help breeders keep track of the genotype and phenotype of related individuals. In humans, pedigrees are often used to show relationships between family members, but can also be used to make predictions about genetic disease inheritance in families.
6. Once students are comfortable with how a pedigree works and its purpose, have students add the information from Activity 2 (top of page 2) to Walter’s cattle herd pedigree.
7. As students work through updating the pedigree, encourage them to write the suspected genotypes of the animals in the pedigree shown. If students need guidance, indicate that working up from the bottom can be helpful and that we may not be able to identify the genotype for every animal in the pedigree.
8. Once students have determined the updated pedigree and the associated genotypes, have students compare their pedigrees to the answers on slides 7 & 8 in the Double the Muscle: Probabilities and Pedigrees PowerPoint presentation. Alternatively, students can collectively build an updated pedigree on the board and fill in genotype information.
9. Have students work independently to show their understanding on questions 7-10 of the worksheet.