Lesson Plan: How did the fly get its eyes?

Students will analyze data to determine the inheritance pattern for eye color in the fruit fly Drosophila melanogaster.

Author Grade Level Content Area
Emily Simpson McDonald 9-10 AP and Honors Biology


Essential Questions

  • Why is it not always possible to determine an organism’s genotype by observing its phenotype?
  • How can you use data to determine how a trait is inherited?
  • Why are males more likely to inherit a recessive sex-linked trait?

Time Needed

Preparation time: 1 hour, one time

Activity time: approximately one 90-minute class period

Post-activity Discussion Time: 15 minutes, in not completed during the same class period as the activity


Bio.3.2 Understand how the environment, and/or the interaction of alleles, influences the expression of genetic traits. 

Bio.3.2.2 Predict offspring ratios based on a variety of inheritance patterns (including dominance, co-dominance, incomplete dominance, multiple alleles, and sex-linked traits).

Making Connections

Prior to this lesson, students should know that an organism’s observable traits, or phenotype, are determined by the combination of alleles it inherits from its parents, also known as its genotype. Students should also know how to use a Punnett square to predict how simple dominant, incompletely dominant, codominant, and sex-linked traits will be passed from parents to offspring. Students should also understand that the results of these crosses are predictions, and that actual phenotypic ratios may differ.


Drosophila melanogaster is the scientific name for the common fruit fly. It is often used as a model organism in biological research because it is easy to work with and its genetics are well documented. Scientists frequently need to analyze phenotype data for fruit fly crosses so that they can determine a fly’s genotype, and in turn, the inheritance pattern of a trait. Like humans, Drosophila is a diploid organism, and it inherits one allele for each gene from each of its parents. The exception to this is for traits with alleles that are on the X chromosome, often called sex-linked or X-linked traits. Female Drosophila have two copies of the X chromosome, and therefore have two alleles for all genes. Males, on the other hand, only have one X chromosome, and therefore have only one allele for sex-linked traits.



Introduction to Drosophila melanogaster

  1. Students read the “Background and Objective” section on the Student Handout.
  2. Teacher projects “Activity Slides” for all students to see.

Teacher goes over slides 1-6 in the “Activity Slides” to provide an overview of Drosophila melanogaster, its genetics, why it is often used in scientific research, and the lesson objective.

Activity: Part 1 - Punnett Square Practice

  1. Teacher assigns students to work in groups of approximately four students. 
  2. Students complete Part 1: Punnett Square Practice of the Student Handout.
  3. Teacher displays slide 7, 8 or 9 of the Activity Slides while students work, depending on which option best meets the needs of the class.
  4. Options for how to complete Part 1: Punnett Square Practice depending on student familiarity of how to use a Punnett square and class time constraints:
    • Slide 7: Each student individually completes Part 1 in its entirety. Group members check each other’s work after all group members have completed Part 1. 
    • Slide 8: Each student in the group completes one section of Punnett Square Practice (either a, b, c, or d). Once all group members have finished their assigned section, all group members check each other’s work and finish completing Part 1.
    • Slide 9: Working in partners within a group of four, one set of partners completes two sections of Punnett Square Practice while the other set completes the other two sections. After both partner sets complete their sections, the entire group of four checks each other’s work and finishes completing Part 1.
  5. Optional: Teacher goes over the results for each section before continuing to the next part.

Activity: Part 2 - Collecting Data

  1. Before the class period: prepare the Fly Cross Bags 
    • For each student group, print off one full copy of the Fly Printouts document single-sided in color and obtain three plastic sandwich bags.
    • For each page in the Fly Print Out documents:
      • Cut out the “label” at the top of the page and tape it to the outside of a plastic sandwich bag
      • Cut out the individual flies on the page and add them to the plastic bag.
    • Alternative: many students are required to obtain in-school service hours for student organizations in which they are involved. Enlist a student needing service hours to complete the tasks outlined in the bullet point above.
  2. Teacher displays slide 10 of the Activity Slides while students work. 
  3. Each student group should obtain a set of Fly Cross Bags (A and B ONLY for Academic students and A, B, and C for Honors students)
  4. Before opening the bags, students should record the parent phenotypes for Cross A under “Part 2: Collecting Data” in their handout. The parents are found on the outside label of the bag.
  5. Only opening the bag for Cross A, students should remove the twenty flies from the bag. These represent the progeny (the offspring) of the parents in the cross.
  6. Students should count the number of females with red eyes, females with white eyes, males with red eyes, and males with white eyes, and record this information in the “Numbers” row of the data table.
  7. Students should then calculate the percentage of each phenotype using the formula below, then record this information in the “Percentage” row of the data table: Percentage = number/20 × 100
  8. Students should clean up by putting the flies back in the bag they came from, ensuring that all twenty flies end up back in the bag and that they all have the same letter as the label of the cross on the front.
  9. Students should repeat the above steps 4-8 for the remaining crosses.

Activity: Part 3 - Analyzing Data

  1. Teacher displays slide 11 of the Activity Slides while students work. 
  2. Students complete Part C: Analyzing Data of the Student Handout
    • Optional: students complete Part C: Analyzing Data for homework, if needed.
  3. After all students have completed all parts of the Student Handout, the teacher facilitates a class Post-Activity Discussion.
    • First, watch the video on slide 12 of the Activity Slides.
    • Then, the teacher facilitates a class discussion while projecting the questions on slide 13 of the Activity Slides. Answers for the questions can be found on slides 14-17.

Wrap Up and Action

The student handout associated with this lesson serves as a formal assessment for student learning. Student learning is also informally assessed using the Post-Activity discussion questions on slide 13 of the Activity Slides. In addition, while this activity is intended to serve as a review of different types of inheritance patterns and an introduction to Drosophila, students can also learn more about Drosophila and their genetics using one or more of the extension activities listed in the next section.


  • Students write a formal lab report or summary of the activity and post-activity discussion.
  • Students observe live Drosophila and perform their own crosses in the classroom. Resources to complete this are listed below:

Guide to Drosophila in the Classroom – Berg Lab, University of Washington


More focused on genetic and laboratory applications beyond the scope of this lesson, but it provides a great overview of the history of the use of Drosophila in research.

About the Author

Emily (Simpson) McDonald is a 2022-2023 Kenan Fellow. She teaches all levels of Biology as well as Anatomy & Physiology at Athens Drive Magnet High School in Raleigh, NC. As an educator, she consistently strives to compassionately innovate connections between content in the classroom and real-world phenomena for her students and colleagues. For her fellowship, she interned in the Hige Lab at UNC Chapel Hill, studying the genetics, behavior, and neurophysiology of the fruit fly Drosophila melanogaster.

About the Fellowship

The Hige Lab at UNC Chapel Hill studies the small, simple brains of fruit flies in order to determine the general circuit principles that underlie the process of memory and decision making. They use flies because their brains are much simpler than those of humans or mice, but they still maintain many of the important sensory circuits and higher-order brain areas.

Furthermore, the lab utilizes genetic tools which label specific types of neurons, or brain cells, allowing them to manipulate and record the activity of fly brains, from a single pair of neurons to an entire brain region. To use these genetic tools, scientists must first determine where the gene labeling the neuron is located within a fly’s genome.

This is done by crossing flies engineered with specific genetic markers with flies that have genes labeling specific neurons. After the flies have been crossed, the phenotypes of their offspring can be analyzed to determine where the gene labeling the neuron is located within the fly’s genome. In this activity, students analyze similar crosses, ultimately determining that the gene for eye color is located on the X chromosome of the fly.

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