Classroom Explorations: Genetic Crosses
Materials & Equipment
Group Size
  • small groups
Preparation
  • Download the images of fruit flies. There’s a separate folder for each fly cross in this activity. Each folder contains the following:
    • images to be projected
    • sheets of small images (the progeny for each cross) to be printed on a color printer and cut into individual images
    • a Punnett square form
    • information regarding the quantities you’ll need
  • Duplicate the student pages.
  • Preview the Drosophila pupae development video (optional).
  • To understand genetic inheritance patterns in the fruit fly.
  • To use Punnett squares to predict the results of genetic crosses and determine the genotypes of the parent flies in a particular cross.
  • You may wish to familiarize students with Drosophila melanogaster by doing the activity Wild Type and Mutant, which uses Drosophila images to introduce students to genetic inheritance. (Even if you don’t do this activity with your students, you might want to read the section called “Primer on Drosophila Notation.” The notation, which is also used in this activity, has been simplified from the standard scientific notation.)
  • Alternatively, review with your students basic genetics terminology (phenotype, genotype, alleles, homozygous, heterozygous), and dominant and recessive inheritance patterns. Students will need to recall—or be introduced to—the inheritance patterns for the following mutations:
The genetics of fruit flies—inheritance patterns and genotypes
Phenotype Inheritance Pattern of Mutation Possible Genotypes

 

wild type

 

N/A

 

+/+, or +/? for recessive mutations

 

vestigial wings

 

recessive

 

vg/vg

 

curly wings

 

dominant

 

Cy/+; Cy/Cy

 

eyeless
(reduced eye size)

 

recessive

 

ey/ey

 

ebony body color

 

recessive

 

e/e

 

white eyes

 

X-linked

Female: Xw/Xw
Male: Xw/Y
Determining genotypes and phenotypes
Organize students into small groups, and lead a discussion based on the following information:
  • We can’t always tell the genotype of an organism from its phenotype. If an organism exhibits a recessively inherited trait, such as vestigial wings in the fruit fly, then we know that it is homozygous recessive (genotype vg/vg). However, when we see a wild-type fly, we don’t know its exact genotype without further testing. It could be homozygous wild type (+/+) or it could be a heterozygote (+/vg).
  • If scientists want to know the genotype of a phenotypically wild-type fly, they can cross it with a fly that shows the recessive phenotype of the trait of interest. The phenotypes of the progeny of this cross will tell us the genotype of the wild-type parent.
Working through an example cross
  1. Hand out the Punnett squares and fly images from the Example folder, and work through the example cross with the students. Each group can fill in its own Punnett square as you go along.
    • Display the wild-type x vestigial flies, image “A” from the Example folder.
    • Invite a student to compare the mutation to the wild-type trait.
    • Ask students if they observe any other differences between the two flies (the one with the darker abdomen is a male).
    • Note: In this case, the female has the mutation—but the gender of the parents doesn’t matter here. Only when a mutation is sex-linked do you have to pay attention to gender.
  2. Draw a blank Punnett square on the board.
      _____ _____
    _____
       
       
    _____
    • A Punnett square provides a way to visualize the genetic makeup of two parents and how the genes recombine to form offspring.
    • The alleles that one parent can contribute are written across the top of the square, and the alleles that can be contributed by the other parent are written along the left side.
    • The possible offspring are represented by the four boxes. Each offspring box receives one allele from each parent. Each allele is written in the boxes in the column or row next to it.
  3. Fill in what is known about the genotypes of the flies in the cross.
    • A question mark indicates an unknown allele.
    • The symbol for vestigial, vg, begins with a lowercase letter, which indicates that the allele is recessive. (If the mutant allele were dominant, the symbol would begin with a capital letter.)
        + ?
      vg
         
         
      vg
    • We know the vestigial fly is homozygous recessive, and we know that the wild-type fly has at least one wild-type allele, but we don’t know what the other allele is—it could also be + or could be vg.
  4. Display image “B” and discuss the following questions, with everyone changing their Punnett squares as you go along:
    • What alleles would the offspring inherit if the wild-type fly is homozygous wild type?
        + +
      vg
      +/vg +/vg
      +/vg +/vg
      vg
    • What are the phenotypes of the offspring? (All are wild type.)
    • What are the genotypes of the offspring? (All are heterozygous [vg/+].)
  5. Have students find pictures that show the phenotypes of the offspring and place the pictures on the Punnett square. Then project image “C,” which shows a Punnett square with pictures of the progeny, and let students compare this to their own Punnett squares.

    Note: The pictures show one male and one female fruit fly in each box, because the probability is that half of the progeny will be male and half will be female; it does not mean that a cross will result in eight flies, four male and four female. In fact, a fruit fly normally produces hundreds of eggs at one time. In addition, note that the percentage of offspring that a Punnett square shows as having a particular genotype reflects probability; the actual results of a cross could vary somewhat.

  6. Discuss the following possibility:
    • What alleles would the offspring inherit if the wild-type fly is a heterozygote? Each small group should fill in a new Punnett square, which should look like the following:
    •   + vg
      vg
      +/vg vg/vg
      +/vg vg/vg
      vg

    • What are the phenotypes of the offspring? (One-half are wild type, and one-half are vestigial.)
    • What are the genotypes of the offspring? (One-half heterozygous [vg/+], and one-half are homozygous recessive [vg/vg].)
  7. Have students add the appropriate pictures to their Punnett squares. Then project image “D,” the Punnett square with pictures of the offspring of this cross, so students can check their work.

Student problems

Display the images for the student problems, giving students time to fill in their Punnett squares and answer the questions on the student pages before showing the next image.

The numbers for each cross listed below correspond to the numbers on the student pages. Each folder contains materials like those for the Example cross. Proceed as in the Example cross—first show the cross without genotypes, then the cross with possible genotypes, then the Punnett square with one possibility, then the Punnett square with the second possibility. Show the images in alphabetical order.

1a. & 1b. Wild type x eyeless
2a. & 2b. Wild type x ebony
2c. F1 cross
3. Wild type x wild type
4a. White-eyed female x wild-type male
4b. Wild-type female x white-eyed male

The cross in this case is curly-winged x curly-winged. Display image “A,” which shows the cross, and challenge your advanced students with this scenario:

You cross two curly-winged mutant flies and notice that you get about one-third wild-type winged flies, and two-thirds curly-winged flies. But there seem to be fewer offspring from this cross than from crosses with curly-winged and wild-type flies. You look in the fly bottle and notice that many pupae—the stage of development after larvae and before adult fly—have not ”hatched.“ Can you come up with any ideas about why this might be so? How would you go about solving this problem?

Ideally, students will draw a Punnett square to help them solve the problem. They might start by showing the dominant curly-winged allele and the unknown second allele for both parents. You can display image “B” to show this:

      Cy ?
    Cy
    Cy/Cy Cy/?
    Cy/? ?/?
    ?

The problem states that the result of this cross is one-third wild-type flies. Students might substitute in a wild-type allele for either or both unknowns to see what results. It’s fine if students try out various possibilities; their experimentation should lead them to discover that only if both parents are heterozygotes can any of the offspring have the wild-type phenotype.

The remaining question is which of the pupae died—what was their genotype? Students might reason that the homozygous wild-type flies should, in general, be healthy, and the heterozygotes should be okay because they have the same genotype as the parents, who are alive and well. That leaves the homozygotes (Cy/Cy) as the probable victims.

You can confirm that the curly-winged allele, when homozygous, is an early development lethal, and display image “C,” which looks like this:

      Cy +
    Cy
    Cy/Cy = LETHAL Cy/+
    Cy/+ +/+
    +

Conclude by congratulating your students on their genetic detective work. Then, if you choose, show the Drosophila pupae development video.

Punnett squares are named for geneticist Reginald Punnett, who developed the square as a tool to help scientists predict the genotypes of offspring. The square is based on probabilities; that is, the likelihood that some event will occur. In the fly crosses in this activity, each square indicates one-fourth of the total number of offspring.


Mutations and gender
Except in the case of the white-eyed flies, all of the crosses shown here have the wild-type phenotype randomly assigned to one gender, and the mutant version to the other gender. We do not show the reciprocal crosses here, but you may wish to do so with your students. There would be no difference in outcome in the reciprocal crosses, except in the case of the white-eyed flies. Since the white-eye mutation is carried on the X chromosome, the gender as well as the trait must be considered in the original cross.

X-linked mutations
Because males have only one X chromosome, we must account for the X and Y chromosomes in our Punnett square. Just as in humans, the results of a cross should always yield half male and half female offspring. Traits carried on the X chromosome, such as white eyes, will be expressed in the male phenotype. For recessive traits, the female would need to have the allele on both X chromosomes to express that trait.

Understanding probabilities
Probabilities are powerful tools for making predictions over a large number of events. The more events you observe, the closer to predicted results the actual results will be. For example, if you flip a coin ten times, probability predicts that the coin will land heads-up five times, and tails-up five times. However, when you actually do this experiment your results could be quite different. If you tossed the coin 100 times, you are more likely to obtain results closer to the predicted fifty percent of each, and tossing the coin 1,000 times will probably give you results even closer to those predicted by probability.

Because fly crosses conducted by scientists and students are set up so that they result in large numbers of offspring, the Punnett square predictions will generally be confirmed by the experimental data. However, with organisms such as humans that produce relatively few offspring, the results are often different than probability predicts. Think about families you know with two or more children. In all cases, are half of the children male and half female? Likely not. However, if you surveyed all the children in each family in your school, your numbers will probably be closer to half and half.

Genetic counseling
Probabilities and Punnett squares are frequently used in genetic counseling. For example, the disease cystic fibrosis (CF) is a recessively inherited disorder that results in the accumulation of thick mucous in the lungs and intestines. In some cases, parents have no idea that they carry the recessive gene for CF because no one in their family has the disease, and they have no symptoms. Probability predicts that when both parents are “carriers” of the CF mutation, they have a one-in-four chance of having a child with CF (you may confirm this with a Punnett square). However, they could have many perfectly healthy children, or all of their children could inherit the CF allele from both parents and develop the disease.