LORD OF THE FLIES: Virtual Fruit Fly Lab Introduction

Fruit flies have long since been ideal organisms to work with while studying genetics. They are easy to keep in a laboratory, have short generation times, and reproduce in large numbers. Fruit flies have 4 pairs of chromosomes, so they are reasonably simple in that regard. These model organisms were the choice of geneticists such as Thomas Hunt Morgan and Alfred Sturtevant; over the last hundred years or so we have learned a substantial amount about eukaryotic genetics from these simple organisms.

In this virtual lab, you will work with several different fly characteristics to, first, replicate the famous experiments of Thomas Hunt Morgan and Alfred Sturtevant. Then, you will determine the inheritance patterns of three other traits by designing and executing your own experiments. You will use a Chi-square Analysis to analyze your data and justify your conclusions.

https://www.sciencecourseware.org/FlyLabJS/

The main screen of the simulation shows how you, the scientist, can design the flies you’d like to cross.

To familiarize yourself with fly terminology, you will examine each design option one at a time. But first, it’s important students understand the definition of “wild-type,” which is an option for each characteristic aside from sex. “Wild-type” is the most common phenotypic expression in nature. So, if you were to catch a fruit fly buzzing about your house, it’s most likely going to be wild-type for all characteristics. Wild-type characteristics are represented with a ‘+’ sign. It’s important to note, though, that wild-type does not necessarily mean that a particular allele is dominant or recessive: it’s the non-mutated, most common version of the characteristic.

The simulation will default to wild-type for each trait unless you choose something else. To choose something else, click the tiny circle to the left of the phenotype you are selecting. To return all characteristics to wild-type, click “reset all” at the bottom.

Click on each of the tabs on the left to explore the different phenotypes for each characteristic.

Let’s practice using some crosses. We will start mono-hybrid crosses, following one trait. For each cross you will examine the F1 generation to determine which allele is dominant and which is recessive, wild type or mutant. You will then mate a female F1 to a male F1 and examine the F2 generation.

The instructions for mating are detailed only for the 1st cross. Follow the steps for other crosses.

Cross # 1

In the simulation, “select fly for mating” a female fly that is wild-type for all characteristics.

Then, under the grayed male fly, click “design fly.” The designer will default to male.

Then, under “wing size” select “vestigial” and then “select fly for mating”.

Double check you’ve not selected something silly (the top should look like this) and then click “Mate Flies”

and then click “Mate Flies”

Your screen will immediately show the results of your cross. NOTE that there are some random processes at play here, so your specific values will not be identical to mine. Here are the results:

Based on YOUR results (not mine) predict which allele is the recessive allele. Justify your prediction.

Based on the answer to question 1, what are the expected ratios for wild type vs. vestigial wing size in the F2 generation. Use a punnett square if you need to.

Experimental Design Review:

In this lab what is your independent variable (what are your changing)?

In this lab what is your dependent variable (what are you measuring as a result of your change)?

To then produce the F2 generation,

1. “Select to Mate” both the male and female,

2. then click “Mate Flies.”

It should give results similar to these (again: slight variations for randomness):

Do the results of your F2 generation support your expected ratios from question #2?

Now, you want to check whether the results obtained “match” your expected results. The simulation has a built-in Chi-Square helper (yay!) but you will still need to do some independent thinking (also yay!). Click on the “Analyze” tab and then check the box toward the top to “Include a test of hypothesis.” If the number of female and male flies with a given phenotype are roughly the same, check the box “Ignore sex of flies” . We will explore this feature later.

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To test the Null hypothesis, you will need to fill out the table that appears with the expected numbers. Using the predicted ratios from above, multiply the total number of flies in the cross. Note this number will be different each time you perform a cross, and different than mine. E.g. if you expected 1:1 ration, you would multiply 1076 by ½ to get the expected # of flies with a given phenotype.48672751201675

On the table it produces, the Chi-squared test statistic is calculated for you, the degrees of freedom are determined, and the level of significance (AKA p-value) is given. Now, before you get all giddy, keep in mind that you will be required to do this sort of analysis on the AP exam using only your calculator and formula sheet, so best make sure you understand what’s going on here before you turn your brain off.

For this reason, you will use the Chi-square value from the table and the degrees of freedom and compare it to the Critical value table below (taken from the Formula sheet you will get on the AP exam) to determine whether you reject or fail to reject the Null.

Make a statement about the Null Hypothesis for Cross #1 (everything you have done with breeding flies so far) and justify it with a comparison between the calculated Chi-Square value and the Critical value for the appropriate degree of freedom.

Cross #2 – Starting with a new set of flies. Mate a wild type female with a male who’s wing size – apterous

Based on your results predict which allele is the recessive allele. Justify your prediction.

Based on the answer to question 5, what are the expected ratios for wild type vs. apterous in the F2 generation. Use a punnett square if you need to.

Make statement about the Null Hypothesis for Cross #2 and justify it with a comparison between the calculated Chi-Square value and the Critical value for the appropriate degree of freedom.

Let’s make it interesting with Cross # 3.

Starting with a new set of flies. Mate a wild type female with a male with white eye color.

Based on your results predict which allele is the recessive allele. Justify your prediction.

Based on the answer to question 8, what are the expected ratios for wild type vs. white eyes in the F2 generation. Use a punnett square if you need to.

Cross the F1 flies to obtain your F2 generation. Paste an image of the results here. What do you notice?

Can you offer a possible explanation of your results?

Write out the genotype of the F1 parents (based on your hypothesis from #8). What cross(es) can you perform with the F1 flies (other than to each other) that would help confirm the genotypes?

You just performed the original Thomas Morgan cross, the one that led to the discovery of gene linkage.