Monohybrid Crosses - Black Body

The following assignment is designed to help you become familiar with the operation of FlyLab and the use of genetic crosses to determine simple inheritance patterns. The FlyLab is a computer program that you interact with over the web using either Netscape or Internet Explorer. The FlyLab simulates the breeding of fruit flies. In the FlyLab you will choose different strains of virtual fruit flies with particular traits (purple eyes, stubby wings, etc.) and mate them (don't worry, they'll be discreet, ). After mating, the program will give you the results by showing pictures of the progeny flies so you can see what traits they have, along with the numbers of each type of progeny that were produced by the mating. From these results and the results of other mating's between the progeny and/or other stock flies, you will determine the rules for the inheritance of the trait, such as recessive, dominant, etc., just as Mendel did with his peas (this is a link to background material).

Specific learning goals for this assignment are:

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

Using the FlyLab

The FlyLab site requires a password which is purchased online at You will use the activation code you purchase to get in and then you will set up your own login and password. Write that login and password down as that is what you will use in the future (you can use the same login and password as for your WebCT account). Be sure to write down your login name and password, you will be needing it frequently throughout the course.

Once your access and Personal User ID and Password are confirmed, go to and click on the FlyLab link.

FlyLab rules and conventions

Before you begin, read the following summary of the FlyLab rules and conventions. This information is also available on the FlyLab site.

(1) There are specific conventions for assigning gene symbols to mutations of the fruit fly Drosophila.

* For a recessive mutation, the symbol for the mutant allele is a lower case letter. For example, the recessive mutation bobbed (short bristles) is symbolized bb and its wild-type allele is symbolized bb+ (= the mutant symbol with a superscript "+"). In Drosophila genetics the term "wild-type" refers to the "normal phenotype found in wild fruit flies (the ones hanging around the fruit at the grocery store).

* For a dominant mutation, the first letter of the symbol for the mutant allele is upper case and the others (if present) are lower case. For example, the dominant mutation notch (wings have a notch in them) is symbolized N and its wild-type allele is symbolized N+ (again, the mutant symbol with a superscript "+").

* However, since users of Virtual FlyLab are supposed to deduce the nature of the genetic mutations by performing crosses, no indication of recessiveness or dominance is given by the symbols used to designate the mutations. Instead, the mutation is abbreviated so that all letters are upper case. In solving the problems, of course, you are expected to use correct genetic symbolism once the genetics is known.

(2) When a mutation is selected, the fly is taken from a "true-breeding" stock (homozygous) unless the mutation is homozygous lethal, in which case it will be heterozygous (think about it for a second and you'll see why).

Monohybrid Crosses - Purple-Eye Example

If you click on this link to the FlyLab it should open the FlyLab site in a separate window. By clicking on the edge of this window you can bring it back to the front and then switch back and forth between the program window and the instruction window as you go through the assignment. If this is difficult for you you might want to print this page so you can look at it while working with the FlyLab. If you have Quicktime this animation will show you how the program is used. Below are detailed instructions for doing crosses and writing a report with a sample monohybrid purple eye cross to familiarize you with using the program and analyzing simple genetic crosses. There is a link at the end of these instructions to a sample report, showing what is expected in a report (no, you don't get any credit for doing this one, and copying and pasting from this report into your report is a bad idea).

Sample investigation of the purple-eye trait (Do NOT write this one up, a sample write-up for this one is posted, this is an EXAMPLE to show you how to do it and what the write-up should look like).

Step 1, loading the program:

  1. Log in to the FlyLab site
  2. Click on the button that says "Start FlyLab" and wait for the program to download to your computer (this may take a couple of minutes). When finished downloading you should see a a gray drawing of two fruit flies.

Step 2, choosing a fly from the purple-eyed strain:

  1. With the mouse pointer, click in the box labeled "Design" that is beneath the female fly (the gray fly on the left should have the word female above it).
  2. You should now see a new screen which has a picture of one fly, in color now, in the lower right part of the window, along with a series of bars along the left side and pictures of different types of bristles along the top. Click on the menu bar on the left that is labeled "Eye Color".
  3. The series of pictures across the top of the window should now show the heads of flies with different colored eyes. Click on the radio button under the head with purple eyes (it will say purple next to the button). The picture of the fly at the bottom of the window should now have purple eyes.
  4. You can click on the other menu options on the left, "body color", etc., to see what traits are available, but do not click on any of the other radio buttons. When you are done looking at the different traits, click on the "Select" button underneath the purple eyed fly.
  5. You have now selected a fly from the purple eye strain and you should be back at the starting screen, only now the female fly is in color and has purple eyes, while the male fly, the one on the right, is still gray. The "Design" button under the female fly has changed to a "Remove" button.
  6. If you have made a mistake and your female fly does not have purple eyes and the wild type (normal) features for everything else (it will say "female: PR" above the fly if you have done everything correctly) then you can click on the "Remove" button to start over at 1 above.

Step 3, choosing a fly from a wild type strain:

  1. Now click on the "Design" button beneath the gray drawing of the male fly.
  2. When you get to the next screen you should notice that the radio button beneath the wild type trait for bristles is already selected. If you go to the "Eye Color" screen, by clicking on the menu bar on the left, you should see that it is also set for the wild type eye color, red.
  3. Leave all of the radio buttons alone and click on the "Select" button beneath the fly. In the future, if you want to get a wild type fly you can just click on the select button as soon as you get to the selection window, as all of the traits start with the wild type selected. Clicking on the "Select" button should return you to the starting screen, only now both flies are in color and the male fly on the right should have red eyes and say, "male:+" above it. The "+" stands for wild type in Drosophila genetics.
  4. If you have something other than a + above your male, click on the "Remove" button, go back to 1 (click on "Design") and make sure that you select a fly with only "+" characteristics.

Step 4, mating the flies:

  1. You should now be back at the starting screen with purple eyed female fly and a wild type male fly. In the earlier steps you have selected a purple eyed female from a stock of purple eyed flies and a wild type male from a stock of wild type flies. Click on the "Mate" button, which should be between the two flies.
  2. This mating between two strains, purple eyes and wild type eyes, is called a hybrid cross.

Step 5, the results:

  1. The screen should update and both of the fly pictures should turn gray again. Below the original flies there is a new window with two a picture of two flies. They should both be in color and have red eyes. The one on the left should have a label above the picture that says, "female: +. N= ~500", the number you see should be somewhere between 450 and 550. The "female" and the "+" identify the picture as being a wild type female fly, the number tells you how many flies like this were produced as a result of your cross. The picture on the right will have a similar label only it will say "male" and the number of males produced will probably be a little different from the number of females.
  2. The progeny produced are called the F1 generation. If you click on the "up arrow", between and a little above the picture of the F1 flies, the window will scroll up to show you the two flies that you mated, these are called the "Parental" generation.
  3. Click on the menu bar labeled "Analyze Results" that is on the left side of the screen.
  4. The window should now show the "Summary of Results". In the main "Summary of Results" view there will be an area at the top labeled "Parents" with a description of the flies used for the cross - in this case it should say (female: PR) x (male: +). Below this should be a table with the heading "Offspring" with the results from the F1 generation.
  5. The table shows the number of each phenotype and sex in the F1 generation, in this case there will be only two rows, one for the wild type females (female: +) and the other for the wild type males. The numbers shown in the "Number" column should be the same as the number that was above the picture of the fly on the earlier screen. The next column shows the proportion of that type of progeny out of the total progeny, it should be approximately .5 for both the males and the females, this just means that about half the flies were female and half were male. The "Ratio" column takes the smallest number from the "Number" column and divides it into the number of each type of fly, thus the smallest number will always get a 1.0 in this column, and, in this case, the other number will be a little larger than 1 (i.e.., 1.012)
  6. As the 1:1 ratio of males to females is not surprising, or of much use for this cross (because there is no differences between the two sexes in how they expressed purple eyes), click on the radio button labeled "Ignore Sex" in the bar above the Results Summary screen. This is not a suggestion for your personal life, but rather produces a new "Summary of Results" window with all of the results for males and females combined. You should do this whenever there is no significant difference between the results for the males and females. Notice that there is now only one row of results with the phenotype as just "+" and the Number around 1,000 with Proportion of 1.0 and Ratio of 1.0. This just means that all of the flies were wild-type.
  7. The radio button labeled "Use Sex" will not be selected now (the black dot in the center is now in the button for "Ignore Sex"). You can use this button to get back to the original "Summary of Results" where the results are divided up by sex. As there was no differences between the sexes for this cross, do not click on the "Use Sex" button.

Step 6, using the notebook:

  1. To the right of the "Use Sex" button is a bar labeled, "Add Data to Notebook". Click the "Add Data to Notebook" bar.
  2. A new window labeled "Notebook" should open in front of the Results window.
  3. The "Notebook" window should open up with the text from the "Summary of Results" window at the top of this new window.
  4. If you start typing on the keyboard the text you type should appear at the bottom of this window, or, by clicking with the mouse in this window you can produce a test insertion point (a blinking ibeam). Use this to type in below the results your observations about the results ("All F1 progeny were wild-type, there was no difference between the sexes"), your hypothesis about how genotype determines phenotype for this trait ("Purple eyes is recessive to wild-type, therefore the genotype of the purple eyed female parent was prpr, the wild-type male parents genotype was pr+pr+ and the genotype of the wild type F1 progeny was pr+pr"), and your prediction for what will happen if you cross two of the wild-type F1 progeny ("I predict that crossing two of the wild-type F1 progeny will produce an F2 progeny with a 3:1 ratio of wild-type to purple eyes"). Include in your analysis a diagram of the cross showing all genotypes and phenotypes. (A phenotypic diagram might be purple x wt --> wt, or F1 purple x F1 purple --> all wt, etc. A genotypic diagram of a cross might be prpr x pr+pr+ --> prpr+, etc.)
  5. After typing in your notes, close the notebook by clicking in the close box in the title bar at the top of the window, or else just click on the main program window to bring it to the front.

Step 7, designing the next cross:

  1. You should now be back at the "Summary of Results" view. Click on the menu bar in the upper left hand corner labeled "Return to Lab".
  2. You should now be at the main view with the pictures of the flies. Make sure that you are looking at the offspring and not the parents (it should say "Offspring" between the two fly pictures) and then click in the select box beneath the female wild-type offspring.
  3. The "Select" Button will gray our and you will now see the color picture of the wild-type female in place of the gray fly above. The "Design" button should also change to a "Remove" button.
  4. Repeat the above procedure with the male offspring.
  5. You have now placed one female offspring and one male offspring in the mating jar. Select the "Mate" button to get the progeny produced by this mating.

Step 8, analyzing the results of the second cross:

  1. You should now see "Cross #2 Offspring" with two pictures of wild-type flies. However, this time there should also be a down arrow between them below the text, "two fly images". Click on this arrow to see the other two fly types produced.
  2. You should now see two images of purple eyed flies.
  3. Click on the "Analyze Results" button to get the table of your results.
  4. As before, choose, "Ignore Sex" and "Add Data to Notebook"
  5. In the notebook, compare your results to your prediction of what the mating would produce. Was your prediction correct? If not, what is different? Do you need to change your hypothesis?
  6. In light of whatever you think is the correct hypothesis, diagram the results of this cross, as you did for the first cross.
  7. Make a prediction for what will happen if you cross one of the purple eyed F2 progeny back to one of the F1 hybrid parents? (a backcross)

Step 9, designing the backcross:

  1. Close the notebook and return to the lab.
  2. Click on the down arrow between the picture of the flies until the purple eyed progeny are showing.
  3. Click the "Select" button beneath the female purple eyed fly.
  4. Now click on the up arrow until you get to the pictures of the F1 wild-type parents. It should say "Cross #2 Parents" between the flies.
  5. Select the wild-type male.
  6. Mate the two flies.

Step 10, analyzing the results and exporting the notes:

  1. Click on "Analyze Results", "Ignore Sex" and "Add Data to Notebook"
  2. In the notebook, compare your results to your prediction of what the mating would produce. Was your prediction correct? If not, what is different? Do you need to change your hypothesis?
  3. In light of whatever you think is the correct hypothesis, diagram the results of this cross, as you did for the first cross.
  4. Write a summary of your investigation with a conclusion about how you believe the purple eye trait is being inherited. Be sure to support your conclusions with results from your crosses. You may want to look at the sample write-up for an example of what I expect as answers to questions like these.
  5. To print our your results, or get an electronic copy for further editing or to e-mail, you need to export the notes. This is because the version of Java used by this simulation does not allow printing.
  6. Click on the "Export Notes" button in the lower left hand corner of the Notebook page.
  7. A new window should open in your browser with a copy of your notes. You can either print this using the browsers print command or save the page as an html page using the save as .. command in your browser. The saved page can be opened in most word processors if you need to do further editing. If exporting is a problem, you can also select all of the text in the Notebook window and paste it into an empty word processor document.

Sample Write-up of Purple Eye Cross

Investigate the Black-Body trait

Cross a wild-type fly with a black-body fly to produce the F1 generation. What does the F1 generation look like? What conclusions can you draw from this result? What do you expect to see in the F2 generation? Propose hypotheses, make predictions, design crosses and diagram the results both phenotypically and genotypically, and draw all appropriate conclusions, as in the purple eye investigation above. Do not worry about proposing hypotheses that turn out to be incorrect, as long as you demonstrate that the hypothesis was false you will be doing fine. You should always test your hypothesis with at least two different crosses ( F1 x F1 and a test cross, F1x homozygous recessive). Reciprocal crosses (mutant male x wt female and wt male x mutant female) are also frequently a good idea. If you write the report in the programs notebook, you should be able to copy the text and then paste it into a word document to turn in. The rubric below should show you how your report will be graded.

Grading rubric to be used to evaluate your report on black body.

1. Student set up crosses, gathered data and tried to interpret the results
Phenotypic descriptions of the crosses and the results are included in the report.

2. After each cross the results are analyzed and there is a clear statement of the current hypothesis

Report has a short paragraph after each cross that discusses whether the results of the cross support the current hypothesis, and, if not, proposes a new hypothesis.
3. All crosses are diagramed both phenotypically and genotypically with clear symbols that reflect the hypothesized inheritance pattern.
Symbols for alleles are used to describe the genotypes along with the phenotypic symbols, and the allele symbols are used correctly to show the type of inheritance and correct number of alleles.
4. Predictions based on the hypothesis are made for each cross (after the first cross) before doing the cross and the crosses chosen are appropriate for testing the current hypothesis.
After analyzing each cross there is a sentence or two describing what the next cross will be, what progeny ratios are expected in the progeny, and why those ratios are expected.
5.Student perseveres and attempts to figure out what is going on even when there is no obvious answer.
If the results do not support a hypothesis it is stated in the report that the hypothesis is false and at least one alternative hypotheses is then tested.
6. There is a proper conclusion to the report.
There is a summary paragraph at the end of the report that states the hypothesis that the student feels is most likely and describes the results that support the chosen hypothesis.
7. Student shows an understanding of all of the genetics principles covered in class
Appropriate hypotheses are tested and are rejected if not supported by the data, and correct allelic and genotypic descriptions are given for P, F1 and F2 flies.

Bell CSU Chico Library
This document is copyright of Jeff Bell
Last Update:
Friday, July 7, 2006