Single Trait Inheritance

Activity 1: Pre-Assessment

1. How would you know if a trait follows a Mendelian inheritance pattern, assuming you know and can track the genotypes and phenotypes of an organism as it produces offspring?
2. What is a Punnett square, and what does it show? Using a Punnett square, predict the offspring of a cross between two heterozygous parents for gene A (Aa × Aa).
3. Discuss the answers to questions 1–2 with the class.

Activity 1: Single Trait Inheritance

Based on the principles set forth by Mendel, we can predict what genotypes and phenotypes offspring will have based on the genotypes and phenotypes of their parents. One efficient way to do this involves using a Punnett square. A Punnett square is a grid where all the alleles of one parent are provided as the column headers while all of the alleles from the second parent are provided as the row headers (Figure 14.1). When the alleles from the two parents are combined in the grid, the internal squares predict the genotypes of their offspring. In addition, multiple Punnett squares can predict offspring genotypes across several generations. The first filial generation (F1) is the offspring that results from crossing the original, parental generation. The second filial generation (F2) is the offspring that results from crossing F1 individuals.

Safety Precautions

• Do not let the flies fly off.
• Inform your teacher immediately of any broken glassware, as it could cause injuries.
• Clean up any spilled fluids to prevent other people from slipping.
• Wash your hands with soap and water after completion of the activity.

For this activity, you will need the following:

• Fruit flies
• Glass vials
• Fly food
• Paint brushes for manipulating flies
• Foam plugs
• Stereomicroscope
• Fly anesthetizing substance

For this activity, you will work in pairs.

Structured Inquiry

Step 1: Hypothesize/Predict: In this activity, you will perform crosses using the fruit fly Drosophila melanogaster. Your teacher will indicate the phenotypes that you will monitor in your crosses and will show you photographs of the traits (e.g., eye color vermillion/sepia, body color gray/ebony, normal wings/curly wings). You will begin by crossing a wild-type parent with a recessive parent. Knowing the dominant and recessive traits of the parent, predict the genotype and phenotype of the first-generation offspring using Punnett squares (see Figure 14.1). Draw the two Punnett squares in your notebook. Add your predictions to the first data table. You will fill the second one with your actual data later.

Step 2: Student-Led Planning: Listen carefully to your teacher’s instructions on how to handle, anesthetize, determine the sex of, and count your flies. Practice, with your partner, how best to use the stereomicroscope. After you understand the procedure, draw a table in your notebook to record your data (sex and phenotype of parent, first generation, and second generation.)

Step 3: Cross the wild-type parent and recessive parents as described by your teacher in Step 1. Then incubate your vials. After the flies have laid eggs, anesthetize and re-collect the parent generation. Remove all the adult flies.

Step 4: After seven days, anesthetize the adult flies from the vials prepared in Step 3. Separate the male and female flies and count the phenotypes present, for each sex. Record your counts in your notebook.

Step 5: Repeat Steps 3 and 4 for the F2 generation. Record your counts in your notebook.

Step 6: Critical Analysis: Are the predictions you made in Step 1 supported by your data? Why or why not? What methods could you use to improve your results? Discuss with your partner and then write your answers in your notebook.

Guided Inquiry

Step 1: Hypothesize/Predict: Predict the original cross and the phenotypic frequencies of an already-crossed group of flies. The phenotypic frequency can be found by dividing the number of flies of a particular trait by the total number of flies. Using flies you already have available, examine the flies and record the sex and traits. Prepare data tables (to record sex and dominant and recessive traits) and Punnett squares.

Step 2: Student-Led Planning: Plan how you will cross your flies. After your teacher has approved, make your crosses for at least two generations. Record your data in your lab notebook.

Step 3: Critical Analysis: Are the predictions you made in Step 2 supported by your data? Why or why not? What methods could you use to improve your results? Discuss with your partner and then write your answers in your notebook.

Assessments

1. You are analyzing the inheritance of a new gene and find that it does not have a clear dominant and recessive allele. Instead, both alleles seem to be expressed when both are present. Does this gene follow a Mendelian inheritance pattern? Why or why not?
2. Answer the following about the assumptions of Mendelian inheritance:
   1. Do most traits in humans follow a Mendelian inheritance pattern? Why or why not?

   2. How could you determine that a trait does or does not follow the assumptions of Mendelian inheritance?