Explore Teaching Examples | Provide Feedback

Monohybrid Fruit Fly Crosses: A Simulation

This page authored by Jeff Bell, California State University, Chico
Author Profile

This material was originally developed through Merlot
as part of its collaboration with the SERC Pedagogic Service.


Students use a computer simulation, the commercial Biolabs Virtual Fly simulation, of fruit fly crosses to investigate the inheritance of a simple Mendelian trait. Students observe the results of initial crosses, make hypotheses about the inheritance of the trait, design crosses to test their hypothesis, analyze the results of their experiment and draw conclusions about the inheritance. This experiment uses a trait with simple dominant and recessive alleles appropriate for any beginning genetics students. A detailed example of how to do the crosses and a sample of a report along with a possible grading rubric are included. This excercise can be easily modified to include dominant, lethal or x-linked alleles.

Used this activity? Share your experiences and modifications

Learning Goals

Scientific Method Learning Goals

  • Propose reasonable genetic hypotheses to explain experimentally derived data
  • Devise experiments to test the hypothesis
  • Evaluate the results of the experimental test and determine whether the hypothesis is supported or not
  • Revise hypotheses to incorporate new results
  • Write a succinct conclusion describing the experimental evidence supporting the most likely hypothesis

Mendelian Genetics Learning Goals

  • Diagram the experimental sequence whereby it can be shown that discrete alternative phenotypes are determined by the alleles of a single gene.
  • Use the principle of equal segregation to predict progeny of crosses of known genotypes for a single gene.
  • From progeny ratios, deduce parental genotypes.
  • Demonstrate the use of branch diagrams and Punnett squares in predicting progeny genotypes and phenotypes.

Communication Learning Goals

  • Be able to write a simple scientific report with observations, hypothesis, description of experiment, results, analysis of results, and a conclusion
  • Demonstrate diagramming a cross showing both phenotypes and genotypes

Context for Use

This assignment is appropriate for any students learning about simple Mendelian inheritance. It requires the use of Internet connected computers so it will work best in a computer lab or with technologically sophisticated students who can use java apps on their own. The actual time needed to do the three or four crosses is only a few minutes and most students that have a basic understanding of dominant and recessive alleles should be able to do the assignment and write-up in an hour. This exercise could either be used in a general biology course as the culminating activity in a unit on inheritance, or as the first of many assignments in a genetics course.

Teaching Materials

Monohybrid-Black-body is an html file that has the instructions for the sample assignment and the actual assignment, along with links to the simulation, an animation of how to use the simulation, and an annotated sample report.

Instructor's notes (Acrobat (PDF) 33kB Jul11 06)

Teaching Notes and Tips

The major difficulty students have with this assignment is understanding how to write genotypes and to use them to keep track of the alleles in a cross. An understanding of how alleles are separated into gametes and then recombined in the next generation is critical to being able to do this assignment. Students who are struggling with Mendelian genetics will tend to only write the phenotypes and ignore the genotypes, or will only give flies one allele for a trait. Some students will also struggle with the fruit fly convention of using a plus superscript on the name of the mutant allele to represent the normal allele (bl+) instead of the more familiar capital and small letters for dominant and recessive alleles used in most introductory text books.

In the write-up, in addition to the problem of not being able to write genotypes or use them to make predictions, many students fail to include a conclusion to their report. This may be because the result is what they expected, so they feel there is no need to repeat it at the end, but if following the form of a scientific report is an important goal of this assignment including a conclusion should be stressed. The assigned cross can be made more difficult than the example by using a mutant trait that is dominant (such as Lobe eye), lethal (Star eye), x-linked recessive (white eye) or even x-linked dominant (bar eye).


The assignment includes a grading rubric at the end based on five points for the assignment; this could be adjusted easily for other courses. I use this assignment in a WebCT course and set the rubric up as a quiz so the students can self-assess their report. I then check their reports and self-assessments to see if they understand what is being asked of them.

References and Resources

MERLOT description of the DNA from the Beginning resource that is used in this simulation.

The first five modules of DNA from the Beginning gives a good description with examples and problems of how to do simple Mendelian crosses such as in this asignment.