Teacher Material: Lesson 2

Teacher Lesson Two
Making crosses!

Students will cross wild type flies and mutants, both purchased from a biological supply house (Carolina Biological Supply or Wards, for example) in groups from lesson one.

Materials:
1. Stereomicroscope
2. Anesthetizing system
3. Paint brushes
4. Culture vials with media
5. Various mutant strains of flies (one or more per group)

Time:
Day 0: Obtain virgin females. This should take 30 minutes, including anesthetizing, or can be assigned earlier in the week.
Day 3: Make F1 cross. Twenty minutes for anesthetizing and selecting females
Day 15-17: Score (count) F1. Make F2 crosses. Allow one 45-minute class period
Day 28-31 Score F2. Thirty minutes for anesthetizing and scoring. Poster presentations: One 45-minute period.

At the heart of Medelian genetics is the observation of phenotypic ratios from mating two phenotypicly dissimilar parents. By analyzing the outcome, specific Medelian ratios, which illustrate a variety of allelic outcomes, including dominance, recessiveness and sex-linked traits, may be observed. Students will obtain visual evidence of genetically inherited patterns via direct manipulation of crosses.

Background information before starting this unit.
Since this is an introduction to Medelian genetics, it is suggested to give a minimal amount of information regarding the outcome. However, there is terminology that they need to acquire. This can be introduced in a classroom setting or as supplemental reading. There are a variety of resources available. One in particular is, "The Monk in the Garden" The chapters entitled "Crossings" and "First Harvest" talk about his initial outcomes, F1 and F2 ratios, and dominance and recessive traits. This would be a great introduction for students to begin.

It is assumed students have an understanding of cell division (mitosis) and the events of meiosis, including crossing over, random assortment of alleles, and chromosomes as trait carriers from generation to generation.

All students need to have completed lesson one before progressing to this lesson.

The use of a Punnett Square is not required to complete this lesson. Instead, introduce it at the end of the lesson. See assessments on incorporating Punnett Squares.

Reference page for additional information on crossing and Punnett Square

Strains
Almost all biology supply catalogs have different strains for crosses. For their first crosses, however, select traits that are easily seen. Refer to the photographs of mutant flies for ideas on easily identifiable traits. Always remember to have backup cultures of flies before handing out to students! Mixing strains of flies is easily done by students. It is not a bad idea for students to have backups also.

Examples of crosses and outcomes.
Sex-linked cross
1. White eye female x wild male F1 males have white eyes F2 ratio of 1 wild: 1 white

Monohybrid
1. Apterous (wingless) x wild F2 ratio of 3 wild: 1 wingless
2. Apterous (wingless) x sepia (sepia eyed)
3. Sepia-eyed female x wild male F2 ratio of 3 wild: 1 sepia

Dihybrid
1. Apterous (wingless red-eyed) x winged sepia-eyed F2 ratio of 9 wild:3 apterous:3 sepia:1 apterous sepia
2. Vestigal winged, red-eyed x winged, sepia-eyed F2 ratio of 9 wild:3 sepia:1 vestigal sepia
3. Vestigial-winged red-eyed female x male winged, ebony-colored bodies F2 ratio of 9wild:3 vestigal:3 ebony: 1 vestigal ebony

Lesson 1
Lesson 2
Lesson 3

Student Material

References