Classroom Explorations: Wild Type and Mutant
Materials & Equipment
Group Size
  • whole class
Preparation
  • Download and preview the Drosophila images.
  • Download the student pages and provide them to the class. If you don’t have a tech center, print out the student pages.
Alternative approach
  • Instead of projecting the Drosophila images, you might print them on a color printer and have students work with them in small groups.
  • To learn about the usefulness of the common fruit fly, Drosophila melanogaster, as a model organism.
  • To understand phenotypes, genotypes, and genetic inheritance patterns.
The following is a concise list of the genetics vocabulary and Drosophila notation used in this activity.

gene A unit of hereditary information consisting of DNA.

allele One of the alternative forms of a gene.

phenotype The traits of an organism that are expressed.

genotype The genetic makeup of an organism.

homozygous Having two identical alleles for a particular trait.

heterozygous Having two different alleles for a particular trait.

dominant allele In a heterozygous condition, the allele that is expressed.

recessive allele In a heterozygous condition, the allele that is not expressed.

wild type An individual having the normal phenotype; that is, the phenotype generally found in a natural population of organisms.

mutant An individual having a phenotype that differs from the normal phenotype.


Primer on Drosophila Notation (simplified from the standard scientific notation)
  • Wild type is designated with a “+” for any allele.
  • Mutations are designated by a letter or letters related to the phenotype of the mutation.
  • Recessively inherited mutations are written in lowercase letters.
  • Dominantly inherited mutations are capitalized.
  • X-linked mutations are written as superscripts to X chromosomes (e.g., Xw). The Y chromosomes are also listed for males.
  • A written genotype lists the two alleles separated by a slash (e.g., +/vg).
The genetics of fruit flies—inheritance patterns and genotypes
Mutation Inheritance pattern Genotypes possible for wild-type phenotypes Genotypes possible for mutants
vestigial wings recessive +/+; +/vg vg/vg
curly wings dominant +/+

Cy/+; Cy/Cy

 

eyeless recessive +/+; +/ey ey/ey
ebony body color recessive +/+; +/e e/e
white eyes X-linked Female: X+/X+; X+/Xw
Male: X+/Y
Female: Xw/Xw
Male: Xw/Y
Discuss the importance of the fruit fly, Drosophila melanogaster, as a model organism, and some of the topics it has been used to investigate:
  • genetic inheritance
  • developmental biology
  • human health problems (e.g., alcoholism)
  1. Have students open the student pages on their computers, or hand out hard copies.
  2. Project the image of the two wild-type flies (image “A”).
    • Have students read the text and answer the questions about the wild-type flies.
    • Lead a discussion about what they observed, then introduce the idea of phenotype.
    • Congratulate any students who realized the flies are male and female. Tell them that the fly with the darker pigmentation at the tip of its abdomen is the male.
  3. Project image “B,” a wild-type fly paired with a vestigial mutant. (Don’t tell them the name of the mutant fly until they have made their observations.)
    • Have students compare the two flies and fill out the description columns in their table.
    • Tell students that the phenotype of this fly is “vestigial” because of its stubby wings, and let them record that.
  4. Repeat #3 with each of the remaining sets of flies.

    NOTE: the final mutant fly, white eyes, is an easy phenotype to see, but the inheritance pattern (X-linked) is best used with advanced students. For an introductory lesson, you may wish to skip this fly.

  5. Discuss with students the idea of dominant and recessive inheritance, and phenotype contrasted with genotype.
  6. Introduce students to Drosophila notation according to the Primer above, or make up your own for use in class.
  7. Reveal to students that the inheritance mode for the first mutation, vestigial, is recessive. If it’s a recessive mutation, what must the genotype of a vestigial fly be? Have the students record this information in their table.
  8. Repeat this procedure with the remaining flies.
    • Remind students that, with a dominant mutation, an individual can have two possible genotypes.
    • If you include the white-eyed fly, students will need to be introduced to the concept of sex-linked characteristics, and they must consider the genotype of both male and female flies.
  9. Discuss this question: If a mutation is recessively inherited, do we know the genotype of a wild-type fly just by observing it? Why or why not?
  10. You might like to copy the What’s Going On? section for your students to read.
For nearly 100 years, the fruit fly Drosophila melanogaster has played a pivotal role in genetics and molecular biology research. In this activity, we have selected fly mutants with easily seen variations and used them as a springboard to help students learn about phenotype, genotype, and genetic inheritance patterns.

Male and female wild-type flies
The male and female differ somewhat in appearance. One difference that can be easily seen in the photomicrographs is that the male has darker pigmentation at the tip of its abdomen. (Other differences are that the tip of the male’s abdomen is rounded while the female’s is pointed, and males have “sex combs,” areas of dark bristles on their front legs that females don’t have. But it’s difficult to see these differences in the images.)

Phenotype and genotype
Phenotype, the physical trait, is determined by the genotype, or genetic makeup of the organism. Single-gene traits are determined by two alleles, one of which is inherited from the mother and the other from the father. A phenotype is a description, whereas the genotype is, in this case, a pair of alleles where each allele may be the same (homozygous, e.g., +/+, vg/vg), or different (heterozygous, e.g., +/vg; Cy/+).

Inheritance patterns
When two copies of the same allele are required to express a particular phenotype, we say that the inheritance pattern for that trait is recessive. For example, the vestigial phenotype is recessively inherited. The genotype of a vestigial fly must be vg/vg. Other recessive mutants in this activity are eyeless and ebony. An example of a human trait that is likely inherited in a recessive fashion is that for widow’s peak (a person’s hairline coming to a point at the top of the forehead). When only one allele is required to express a trait, as is the case with the curly-winged mutation, its inheritance pattern is dominant. The genotype of a curly-winged fly could be Cy/+ or Cy/Cy. An example of a dominantly inherited trait in humans is that for achondroplasia, a form of dwarfism.

Genotype of a wild-type fly
When we observe a fly that is wild type in appearance, and we’re considering its genotype, we don’t really know if it’s homozygous or heterozygous for a recessive mutation. It may carry one allele that is wild type, for example, for body color, and one that is recessive, for example, the ebony allele. Because ebony is inherited recessively, we know that the wild-type fly must have at least one allele that is wild type for body color. We could discover its genotype by doing a genetic cross with a recessive homozygote, in this example, an ebony fly. This idea is covered in the activity Genetic Crosses.

X-linked mutations

The white-eyed mutation was the first fly mutation discovered. It is an X-linked, or sex-linked, mutation. As in humans, flies that carry two X chromosomes are female, and flies that carry one X and one Y are male. In fruit flies, the Y chromosome is structurally different from the X chromosome, and it doesn’t carry genes that are complementary to those on the X, so any gene that is on the X in a male will be expressed, while the regular rules of dominant and recessive inheritance apply to female flies because they carry two X chromosomes. A white-eyed male must have the white mutation on its single X chromosome. In a female fly, the white mutation is inherited recessively, so two copies of the white mutation are necessary to produce a white-eyed female.