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Genetics with Drosophila F1 Crosses

Drosophila is an excellent model organism for the study of genetics

Carolina Labsheets™

Drosophila is one of the most popular model organisms for the study of genetics. One hindrance to using Drosophila, however, is the need to select virgin female flies for crosses. The use of our Drosophila F1 Crosses removes this obstacle. Our technicians select virgin female flies and set up the crosses to produce the F1 cultures that we ship. The F1 flies that emerge in these cultures can only mate among themselves, so your students need only to collect F1 flies and set them up in fresh vials to produce an F2 culture. Each F1 culture can be used to set up about eight vials to produce F2 flies.

The following F1 autosomal crosses are available. The phenotypes used for these crosses are easily distinguishable from the wild type. They are also widely used in lab manuals and textbooks. Since the crosses must be set up before shipment, please give a 2-week notice for delivery of the cultures. Otherwise, the crosses may not be available when you need them. We can also do custom F1 crosses (172055). Select the parents from our general listing of Drosophila strains and notify us 6 weeks before the desired delivery date.





Monohybrid Crosses

172000 F1 apterous x wild

A vial of F1 flies from a cross between wingless (172320 apterous) and winged (172100 wild type) parents. Apterous (ap) is recessive and is located on chromosome 2. The expected F2 ratio is 3 wild type: 1 apterous.

172010 F1 sepia x wild

A vial of F1 flies from a cross between dark brown-eyed (172575 sepia) and red-eyed (172100 wild type) parents. Sepia (se) is recessive and is located on chromosome 3. The expected F2 ratio is 3 wild type: 1 sepia.

Dihybrid Crosses

172030 F1 apterous x sepia

A vial of F1flies from a cross between wingless(172320 apterous) and dark brown-eyed (172575 sepia) parents. Apterous (ap) is recessive and is located on chromosome 2. Sepia (se) is recessive and is located on chromosome 3. Apterous flies are wild type for eye color, and the sepia flies are wild type for wings. The expected F2 ratio is 9 normal wings, red eyes: 3 normal wings, sepia eyes: 3 apterous, red eyes: 1 apterous, sepia eyes.

172032 F1 vestigial x sepia

A vial of F1 flies from a cross between reduced wings (172460 vestigial) and dark brown-eyed (172575 sepia) parents. Vestigial (vg) is recessive and is located on chromosome 2. Sepia (se) is recessive and is located on chromosome 3. Vestigial flies are wild type for eye color, and the sepia flies are wild type for wings. The expected F2 ratio is 9 normal wings, red eyes: 3 normal wings, sepia eyes: 3 vestigial, red eyes: 1 vestigial, sepia eyes.

172035 F1 vestigial x ebony

A vial of F1 flies from a cross between reduced wings (172460 vestigial) and dark-bodied (172500 ebony) parents. Vestigial (vg) is recessive and is located on chromosome 2. Ebony (e) is recessive and is located on chromosome 3. Vestigial flies are wild type (tan) for body color, and the ebony flies are wild type for wings. The expected F2 ratio is 9 normal wings, normal body color: 3 normal wings, dark body: 3 vestigial, normal body color: 1 vestigial, dark body.

For sex-linkage, see our Genetics with Drosophila F1 Crosses: Sex-Linkage Labsheet.

Needed Materials

F1 Drosophila cultures

empty culture vials and plugs (173076)

vial labels or wax pencils

Formula 4-24® Instant Drosophila Medium (173200)

FlyNap® Kit (173010)

alcohol

Drosophila sorting brushes (173094)

index cards to use as sorting trays

Carolina™ Drosophila Manual (452620)

stereomicroscopes

Optional Materials

Carolina Drosophila Stand (173030)

Our Drosophila Culture Kit (173050) includes 36 vials and plugs, labels, FlyNap® Kit, Formula 4-24®, sorting brushes, sorting cards, and a Carolina™ Drosophila Manual.

Drosophila Care Guide

Safety

Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as safety glasses or goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules.

Procedures

Students can work individually or in groups of 2–4.

When your cultures arrive, open the package immediately and inspect the cultures to verify that they have arrived in good condition. Examine the label on each vial. The label is divided into four parts. The upper left quadrant gives information on the female flies used for the cross, and the upper right quadrant gives information on the male flies. The lower left quadrant is stamped with the date that the parent flies were placed in the vial. F1 flies should begin emerging 12–14 days after the date on the label.

Each culture will produce approximately 100 flies over a 10-day period, and will serve 6–8 groups.

Either prepare a vial with medium for each student group or provide materials and instructions to the students so they can prepare their own vials. Materials needed are culture vials with plugs and Formula 4-24® Instant Drosophila Medium. Open the bag of medium and locate the small measuring cup and packet of yeast. Add a level cup of medium to each vial. When all the vials have dry medium, add a level cup of cool water to each vial. Open the yeast packet and sprinkle a few (6–8) grains of yeast onto the surface of the medium in each vial. Do not add too much yeast. The yeast produces CO2, which in large amounts can cause sterility or even death of the flies.

Either prepare the fly morgues with alcohol (one morgue is included in each FlyNap® Kit), or provide materials and instructions so the students can do this themselves.

Observing F1 Flies and Setting Up F2 Cultures

Anesthetize flies before the lab and dump them onto sheets of paper. When the FlyNap® wand has been inserted into a vial, either place the vial upside down on a Carolina Drosophila Stand or lay the vial on its side to prevent the flies from dropping onto the medium and possibly becoming stuck. At the beginning of the lab, students will pick up the flies on index cards.

Students will also need the following materials for doing initial observations and setting up cultures for F2 flies:

  • vial with medium and plug
  • vial label
  • FlyNap® Kit (if flies are to be anesthetized by students during lab; one for every 6–8 groups of students)
  • sorting brush
  • index card
  • stereomicroscope

Since the students do not have to select female flies of one strain to mate with males of another, the lab does not ask them to practice sexing the flies. If you would like students to practice sexing of flies, refer to page 12 of the Carolina Drosophila Manual and have them do that as part of the Observing activity.

Clearing F1 Parents from the F2 Vials

Seven to 10 days after the F2 vials are set up, the F1 flies need to be removed from the F2 cultures. This is done to avoid including the F1 flies in the F2 counts.

The following materials are needed:

  • FlyNap® Kit (one for every 6–8 groups of students)
  • sorting brush
  • index card

Scoring Phenotypes

F2 flies will begin emerging about 12–14 days after students have set up their vials. Begin scoring phenotypes of the F2 on the day after the flies first begin emerging. More females than males emerge on the first day, but the ratio evens out on successive days. Scored flies should not be returned to the vial; they must be disposed of in a morgue to prevent them from being counted twice. Phenotypes can be scored every other day for up to 10 days. Counts carried beyond 10 days risk including flies of the next generation.

For scoring the phenotypes of the F2 flies, students will need their F2 vials and the following materials:

  • FlyNap® Kit (one for every 6–8 groups of students)
  • sorting brush
  • index card
  • stereomicroscope

Optional

As an option, you can provide cultures of the parent flies of the cross. Students can compare the phenotypes of the parental flies to the F1 culture to see which is dominant. The activity can be made more investigative by removing the label from the F1 culture vial and asking the students to analyze the cross without the information provided on the vial. For a demonstration of the use of FlyNap®, students can watch our Observing Phenotypes with Drosophila melanogaster video.


Chi-square analysis is not included as part of the activity, but it can easily be added. There are several Internet sites that will perform the calculation from input data.

Answer Key to Questions Asked on the Student LabSheet

Note the eye color and wing type of your F1 flies here. Note: if you are using the 172035 F1 vestigial x ebony, change "eye color" to "body color" for the phenotype.

Eye color  wild type

Wing type  wild type


1. What is the genotype of the F1 flies?

Heterozygous.

2. Which phenotype(s) result from the action of a dominant allele(s). How do you know?

The wild-type alleles are dominant. Their phenotypes are expressed in the heterozygous F1.

3. What are the expected phenotypes and their ratios in the F2?

Answers will vary according to the cross used. Ratios will be 3:1 for monohybrid crosses and 9:3:3:1 for dihybrid crosses.

4. Why is it necessary to remove the F1 flies?

So that they will not be counted with the F2 flies that emerge in the vial. (Some students may think that it is because the F1 flies might mate with the F2; however, since the activity will end before the F3 flies emerge, this is not of much concern.)

5. Are your results compatible with your expected ratios? Explain.

Answers will vary according to data collected. Students can run a chi-square on their data, both for individual groups and for the class. There are websites that calculate chi-square from input data.

Sample Data for a Monohybrid Cross

F2 Phenotypes and Numbers of Flies

Date Counted Phenotype 1 Phenotype 2 Phenotype 3 Phenotype 4
         
         
         
         
         
         
Total 362 133    
Expected Ratio 3 1    

Sample Data for a Dihybrid Cross

F2 Phenotypes and Numbers of Flies

Date Counted Phenotype 1 Phenotype 2 Phenotype 3 Phenotype 4
         
         
         
         
         
         
Total 579 206 178 61
Expected Ratio 9 3 3 1









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