Paleobio Home Cassit Birds: Lessons on Genes, Inheritance,and Evolution
Lesson Three

Written by Jennifer A. Collins
Jen's Other Lessons   ~  Education
overview  ~   one  ~   two  ~   three  ~   four  ~  
Paleobio Home
Concept: Offspring inherit half their genes from a female and half from a male through sexual reproduction. Gene frequency and phenotype frequency fluctuate from generation to generation.

Overview: Students identify that gametes have only one copy of each chromosome compared to the two copies found in somatic cells. To understand how phenotypes of offspring are determined, students are given their own gamete with genes labeled. They simulate fertilization by finding a "mate" and determine the genotype and phenotype of the offspring produced. Students then use their understanding of sexual reproduction to create possible gametes for their own Cassit Bird, and mate them with that of another classmate to produce two offspring. These birds become part of the F1 generation. After graphing this new data, students can see how the population has changed in gene and phenotype frequency.

Time:40-60 minutes

Grouping:partners, class

Vocabulary: phenotype, genotype, homozygous, heterozygous, allele

Advance Preparation: Make overheads, cut out Cassit Bird Sex Cells so that each student has one sex cell.


  • Cassit Bird Gametes Introduction
  • Cassit Bird Gametes (enough for 1 cell per student. Half the students should have male gametes and half should have female gametes)
  • Mating Rules(overhead)
  • Our Offspring Female
  • Our Offspring Male
  • Cassit Bird Data Table and graph

    Procedure: 1. Show students the Cassit Bird Gametes Introduction.
    Ask students:

  • This is a sperm from a male bird. What type of sex cell do the female birds have? (egg).
  • What are the similarities and differences between the body cell and the sex cell? (The sex cells have half the number of chromosomes as the other cells. The body cells have matching chromosomes).
  • Why do you think there is only half the chromosomes/genes in the sex cells? (Half the genetic material comes from a female, half from a male when fertilization occurs so that the offspring have pairs of chromosomes to code for their phenotypes. Discuss why it might be disadvantageous if meiosis did not occur).

    2. Pass out ONE of the Cassit Bird Gametes to each student. Explain that each of them has a sex cell from either a male or female Cassit bird. On the cells is the labeled genes for the features discussed.

    3. Remind students that Cassit birds reproduce sexually which means that eggs from a female join with sperm from a male. The combined result of the cells is fertilization in which the resulting cell is made up of half of the female's DNA and half of the male's DNA.

    4. Have all students stand up. Each student needs to find one other bird to "mate" with, thus anyone with a sperm must find an egg and vice versa. Once the students have found a mate, they need to figure out what the offspring looks like, both the genotype and phenotype. Have students write their results on paper, or have students share their results with the class.

    5. Repeat this process several times so students get the idea of how egg and sperm carry half the genetic material. When fertilization occurs the genetic material joins to form gene pairs which code for the phenotype.

    6. Tell students that they will now do this with the class population of Cassit birds. The Cassit Bird that they created (from My Cassit Bird) begins to produce gametes when it matures. The gametes that are produced can have different alleles (as long as the genes are found in the adult). Show this idea with an example. (In this example, both the male and female produce multiple gametes at a time).

    7. Have students take out their My Cassit Bird. At the bottom of the handout are 5 gametes. Students need to write in five different combinations of alleles that might be produced by their bird. Emphasize that the genes they fill in must exist in their adult bird. Note: You could also have students show how their cells would undergo meiosis to produce various combinations of genes.

    8. Explain to students that they will be creating the next generation of birds. Put up the Mating Rules. Discuss. Before they begin, students must cut out the sex cells they created on their My Cassit Bird sheet. When they cut out the cells make sure they have a safe place to keep them. NOte: You can alter the "Mating Rules" to include directions to have students make Punnet Squares to show the probability of various genotypes and phenotypes.

    9. Once all students have cut their cells out, explain to students that they will need to find a mate to reproduce. Have students get up and find a bird of the opposite sex.

    10. Students should turn all of their sex cells upside down and randomly draw one cell from the female's pile and one from the male's pile. This simulates fertilization. They should study the genes that their first offspring has inherited and complete an Our Offspring ( female and/or male) for each offspring produced.

    11. Once all students have created two offspring, the class should fill in the data for this next generation on the Cassit Bird Data Table and add to their graph. (Students create two offspring so that each student can have a bird to create the next generation.)

    12. Repeat this same process for one or more generations. You will need to "kill off" some of the birds so that the population size does not grow exponentially. This needs to be done RANDOMLY so that you are not demonstrating some type of selection. For example you could eliminate all of the parent population birds once they have created their offspring.

    13. Once students have completed their graphs, discuss the following questions:

  • Look closely at the graph that you created. Explain how each phenotype has changed or not changed through time (from generation to generation).
  • What would you expect the population to look like in the future?
  • What would you expect the population to look like if, for some reason, birds with short/fat beaks were unable to get enough food?

  • ©   Updated: June 2003   Contact: jen-AT-paleobio or allen-AT-paleobio