Teacher Notes

Sno-ball Sillies—Genetics Simulation

Student Laboratory Kit

Materials Included In Kit

Chenille wires, 10
Corks, Size 00, 45
Dad chromosomes, blue, 16 sets
Mom chromosomes, pink, 16 sets
Plastic bags, 30
Pop beads, 100
Push pins, clear, 1 box
Push pins, colored, 1 box
Screws, black, 16
Screws, silver, 16
Styrofoam® balls, 1–½", 60
Toothpicks, plastic, 50
Toothpicks, wooden, 75

Additional Materials Required

Scissors (for Prelab Preparation)

Prelab Preparation

  1. Cut out enough sets of the pink (MOM) chromosomes for each student group and place into separate reclosable plastic bags. Label each bag “MOM.”
  2. Cut out enough sets of the blue (DAD) chromosomes for each student group and place into separate reclosable plastic bags. Label each bag “DAD.”
  3. Cut each chenille wire (tail) in half. There will be a total of 20 tails available for use.
  4. Photocopy enough Sno-ball Sillies—Genetics Simulation Decoder worksheets for each student group to use after they have determined the offsprings’ genotypes.

Safety Precautions

Remind students to use caution when handling sharp pins. Please follow all laboratory safety guidelines.


Offspring can be dismantled and all items may be saved for future use or disposed of in the regular trash.

Lab Hints

  • Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation may be completed the day after the lab.
  • Copying and cutting out extra chromosomes is a good idea in case students lose or misplace chromosomes. This will prevent delay in future class periods. One extra set is provided.
  • There are enough materials to build a “MOM” and “DAD” Sno-ball Silly to have on display in the classroom.

Teacher Tips

  • This activity is ideal as a review of heredity, meiosis, dominant and recessive traits, alleles, chromosomes and Punnett squares.
  • A test cross is a breeding experiment with an organism of an unknown genotype associated with the dominant phenotype. This organism is mated to an organism that possesses the homozygous recessive phenotype. Based on the offspring, the unknown genotype can be inferred.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Using mathematics and computational thinking
Developing and using models
Analyzing and interpreting data

Disciplinary Core Ideas

MS-LS3.A: Inheritance of Traits
MS-LS3.B: Variation of Traits
HS-LS1.B: Growth and Development of Organisms
HS-LS3.A: Inheritance of Traits
HS-LS3.B: Variation of Traits

Crosscutting Concepts

Structure and function
Cause and effect
Systems and system models

Performance Expectations

MS-LS3-2: Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.
HS-LS3-1: Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring.
HS-LS3-2: Make and defend a claim based on evidence that inheritable genetic variations may result from (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.
HS-LS3-3: Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.

Answers to Prelab Questions

  1. Each diploid cell of the new species contains 16 homologous chromosomes. How many chromosomes are present in each haploid cell?

    Each haploid cell of the new species would have 8 chromosomes.

  2. Upon fertilization, how many chromosomes will the offspring possess?

    After fertilization, the number of chromosomes would return to the diploid number of 16.

  3. Use a diagram to explain how the process of meiosis produces four haploid cells.

    The process of meiosis has two cellular divisions. In the first division, homologous chromosomes with two sister chromatids are separated reducing the number of chromosomes. In the second division, sister chromatids are separated just as they are in mitosis.


Sample Data

Offspring Sketch

Answers to Questions

  1. How many unique offspring were created in the class?

    Data will vary. It is probable that there will be 15 unique offspring.

  2. If any two were exactly alike, did their genotypes match also?

    Data will vary. It is unlikely that genotypes and gender will be identical.

  3. Compare the offspring to the parents.
    1. Do any of the offspring look exactly like either of the parents?

      Student answers will vary, however, it is unlikely that an offspring will be identical to one parent.

    2. What would happen if it were possible for an offspring to inherit all of its chromosomes from one parent?

      If an offspring inherited all of the chromosomes from one parent, it would be an identical copy or a clone.

  4. Choose another team’s offspring to be a mate for your model. Select two of the traits and complete a Punnett square for each.

    Student answers will vary. Example:

  5. The following table includes the phenotypes of each parent. Using the class data of offspring, determine the genotypes for each parent’s traits.

    *Correct genotype based on chromosomes.

  6. Is it possible for a mating pair of two-eyed Sno-ball Sillies to have offspring with three eyes? Explain your reasoning.

    It is possible. If the two-eyed Sno-ball Sillies are both heterozygous for number of eyes, then even though they both have two eyes, they possess the allele for three eyes; it is just hidden by the dominant allele. If the offspring received the recessive allele from each parent, the offspring would have three eyes.

  7. By random selection of one of two alleles for each of the eight traits, how many different varieties of offspring can be created? (Hint: If two forms for a trait exist, the possibilites are 2 × 2 = 4; if three traits exist, the possibilities are 2 × 2 × 2 = 8).

    28 = 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 = 256 different varieties could be created.

  8. If none of the offspring had three body segments, what might be inferred about the DAD’s genotype for body segments? Can you be certain?

    It could be inferred that the DAD’s genotype is homozygous dominant (BB); however, it cannot be certain. It is possible the DAD’s genotype is Bb and the dominant allele was randomly selected for all the offspring.

Teacher Handouts


Student Pages

Sno-ball Sillies—Genetics Simulation


As a naturalist, a student of natural history, you are studying organisms of the Amazon Rainforest. On an expedition, you discovered a colony of a new species—dubbed the “sno-ball sillies” due to their snow ball-shaped bodies—which has yet to be identified. Wanting to know as much as you can about this new species, you bring a male and female back to your research lab to breed them and learn about their traits.


  • Alleles
  • Genotype vs. phenotype
  • Dominant and recessive
  • Chromosomes
  • Meiosis


The discovery of a new species is a time of excitement and questions. For example, Microhyla laterite, a new species of frog discovered in Manipal, India, in 2016 led researchers on an exhilarating adventure to determine whether this tiny frog, which can sit on the tip of your thumb, was in fact, a new species. Finally, through genetic analysis it was confirmed to be an unknown species. The frog’s scientific name came from its narrow mouth (Microhyla) and the habitat where it was found, laterite—a rocky terrain of iron-rich, weathered soil.

Examining an organism at the cellular level shows that almost all cells have the same number and type of chromosomes. For example, a human body cell has 46 chromosomes. Each chromosome matches up to make a pair that is similar in shape and size. These are called homologous chromosomes (see Figure 1) and are inherited from the parents. One is inherited from the mother and one is inherited from the father. Each homologous chromosome in a pair carries the same sequence of genes which encode for traits. However, the version of the gene, called an allele, found on one homologous chromosome does not always match the other. Alleles generally are either dominant or recessive. A genotype represents the alleles contained in the gene of a homologous pair and can only be determined through laboratory testing, whereas a phenotype is the observable characteristic. A dominant phenotype only requires the presence of one allele in order for the trait to be observable, regardless of the other allele present. Recessive phenotypes require the presence of two copies of the same allele. For example, the gene for freckles is on chromosome 16 (see Figure 1a). The alleles present determine whether or not a person has freckles (dominant) or no freckles (recessive). Looking at Figure 1, one chromosome carries the dominant allele (F) for freckles and the other carries the recessive allele (f) for no freckles (see Figure 1b). In this person, freckles will be seen in the phenotype because the dominant allele hides or masks the recessive allele.

Humans have 23 homologous pairs of chromosomes. In females, all 23 match in size and shape. In males, however, one pair does not match. The two chromosomes that do not match are the X and Y, or sex chromosomes. Not all species follow this pattern. For example, in birds, snakes and some insects, females carry the mismatched chromosome pair while males carry the identical pair.

A cell that contains two homologous sets of chromosomes is knows as a diploid cell. The total number of chromosomes is known as the diploid number. In humans, a diploid cell has 46 chromosomes. Sex cells, or gametes, are not diploid cells. Sperm cells and egg cells each have a single set of chromosomes, one from each homologous pair. They are known as haploid cells and are produced through the process of meiosis.

Meiosis is the type of cell division that occurs in reproductive tissues. During meiosis two cellular divisions occur. In the first division, homologous chromosomes with two sister chromatids are separated, reducing the number of chromosomes (see Figure 2). In the second division, sister chromatids are separated, just as they are in mitosis. During meiosis, the cells reduce their normal diploid chromosome number by half to create four haploid cells. By reducing the number to half, when fertilization (fusion of nuclei and cytoplasm from gametes resulting in a zygote) occurs, the number of chromosomes in the offspring return to the diploid number. Each offspring would have two homologous sets of chromosomes, one from each parent. Having haploid cells fuse rather than diploid cells prevents the doubling of chromosomes between generations, which would be detrimental.

Experiment Overview

Use your knowledge of meiosis and heredity to create an offspring from a mating pair of the new, unknown species. From the offspring created, determine the parents’ genotypes.


Chenille wire
Plastic bag labeled DAD
Plastic bag labeled MOM
Pop beads
Push pins, clear and colored
Screws, black and silver
Styrofoam® balls
Toothpicks, plastic
Toothpicks, wooden

Prelab Questions

  1. Each diploid cell of the new species contains 16 homologous chromosomes. How many chromosomes are present in each haploid cell?
  2. Upon fertilization, how many chromosomes will the offspring possess?
  3. Use a diagram to explain how the process of meiosis produces four haploid cells.

Safety Precautions

Pins are sharp; handle with care. Please follow all laboratory safety guidelines.


Part A. Chromosome Sorting

  1. Take the pink chromosomes out of the plastic bag labeled “MOM.”
  2. Put the chromosomes on the lab table with the letters face down.
  3. Match the chromosomes as homologous pairs (matching size).
  4. Randomly take one chromosome from each homologous pair. Place chromosomes NOT chosen back in the plastic bag.
  5. Repeat steps 1–4 with the blue chromosomes in the plastic bag labeled “DAD.”
  6. Match the “MOM” chromosomes to the homologous “DAD” chromosomes (match size).
  7. Flip the chromosomes over and fill in the table on the Sno-ball Sillies Genetics Simulation worksheet.
  8. Return all the chromosomes to the correct plastic bags.
  9. Obtain a Sno-ball Sillies Genetics Simualtion Decoder sheet from the instructor and determine the phenotype of the offspring. Fill in the correct phenotype on the worksheet.
Part B. Offspring Building
  1. Using the materials provided, assemble the offspring according to the genotype selected in Part A.
  2. Use wooden toothpicks to hold the body segments together and attach the humps. The toothpicks may be broken in half if needed.
  3. After the offspring is assembled, draw the offspring on the worksheet. Used colored pencils or label the appropriate color when necessary.
  4. Record the phenotype of each trait for the offspring on the class data worksheet on the board.
  5. Fill in the Class Data table on the worksheet in your packet. This will be needed for the Post-Lab Questions.
  6. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF


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