Modeling Genetic Variation in Gametes
PSI AP Biology
Name: _________________________
Objective
Students will model the processes of gamete formation that increase genetic variation: independent
assortment, crossing-over, and fertilization.
Standard
Essential Knowledge:
3A2c : Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually
reproducing organisms.
3C1c : Errors in mitosis or meiosis can result in changes in phenotype.
3C2c : Sexual reproduction in eukaryotes involving gamete formation, including crossing-over during
meiosis and the random assortment of chromosomes during meiosis, and fertilization serve to
increase variation. Reproduction processes that increase genetic variation are evolutionarily
conserved and are shared by various organisms.
Materials
Every student needs:
Activity worksheet
Every student pair needs:
12 toothpicks
18 mini marshmallows of one color
18 mini marshmallows of another color
Scissors
Ziploc bag
Procedure
1. Read through the activity worksheet. Instructions for each activity are given within the
worksheet.
2. Answer the Analysis and Application questions.
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Gamete Formation
Gamete formation results from meiosis, which is split into two stages: meiosis I and meiosis II.
Meiosis I
Function:
Separate
homologous
chromosomes
Diploid cell with
duplicated chromosomes
Meiosis II
Result:
Haploid cells
with
duplicated
chromosomes
Haploid cell with
duplicated chromosomes
Function:
Separate sister
chromatids
Result:
Haploid cells
with
unduplicated
chromosomes
Haploid cell with
duplicated chromosomes
Haploid cell with unduplicated chromosomes
Gamete formation involves three processes that increases genetic variation:
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1. Independent assortment
Independent assortment occurs at metaphase I. At this point, chromosomes are diploid and
duplicated. Independent assortment refers to the fact that the pairs of chromosomes line up at
the metaphase plate independently of each other.
2. Crossing-over
Crossing-over occurs during prophase I. At this time, DNA from two nonsister chromatids is
switched, resulting in recombinant chromosomes.
3. Random fertilization
During fertilization, one male and one female gamete pair together randomly. By this point,
there is enough variation that subsequent fertilization events from the same male and female
will all produce unique offspring.
Modeling independent assortment
As a human, you are a diploid organism with 23 chromosomes. You received 23 from your mother and
23 from your father at fertilization. Independent assortment refers to the fact that, during gamete
formation, your chromosomes match up in a variety of ways. Your gametes are not composed of all your
maternal chromosomes or all your paternal chromosomes. Some gametes will have 2 paternal and 21
maternal, 13 paternal and 10 maternal, 19 paternal and 4 maternal, and so on. Your chromosomes sort
themselves into haploid gametes independently from each other.
Procedure
1. Gather your supplies:
 12 toothpicks
 18 mini marshmallows of one color
 18 mini marshmallows of another color
2. You will be modeling a diploid organism with 3 chromosomes. Chromosome A has 4 genes;
chromosome B has 3 genes; chromosome C has 2 genes.
3. Using the 18 marshmallows of the first color, build the maternal chromosomes:
 On 2 toothpicks, slide on 4 marshmallows each. This represents chromosome A.
 On 2 toothpicks, slide on 3 marshmallows each. This represents chromosome B.
 On 2 toothpicks, slide on 2 marshmallows each. This represents chromosome C.
4. Using the 18 marshmallows of the second color, build the paternal chromosomes following the
same pattern as listed in step #3.
5. You should now have the same chromosomal representation as the cells that are starting
meiosis: diploid with duplicated chromosomes.
6. The result of meiosis is haploid cells with unduplicated chromosomes. Using your chromosomal
representations, how many different combinations can you come up with to represent daughter
cells of meiosis? (Hint: paternal A, paternal B, paternal C counts as one combination. How many
others can you come up with?)
Analysis
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1. In an organism with 3 chromosomes, how many possible daughter cells are there?
2. When there are 2 homologous chromosomes, there are 4 different possible daughter cell
combinations. When there are four homologous chromosomes, there are 16 different possible
daughter cell combinations. Based on this information, create a mathematical model that
relates the number of chromosomes to possible daughter cells.
Modeling Crossing-over
While independent assortment creates vast variation in daughter cells, crossing-over increasing that
variation even more by mixing up pieces of maternal and paternal chromosomes into recombinant
chromosomes.
Procedure
1. Use the same organism information and chromosomes from the independent assortment
activity.
2. Pair the duplicated homologous chromosomes next to each other. (You should have 3 sets of 4
toothpicks.)
3. On the two chromatids that are in the center of each pairing, switch the top marshmallows.
4. Separate all the chromosomes so that they represent the unduplicated chromosomes found in
daughter cells.
5. Do you have any identical chromosomes anymore?
Analysis
1. Describe how crossing-over creates variation.
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Modeling random fertilization
In an organism with 4 chromosomes, there are 16 different combinations possible for the gametes.
During fertilization, 1 out of the 16 combinations is randomly chosen from the male organism and 1 out
of the 16 combinations is randomly chosen from the female organism. The resulting zygote will be a
unique organism with a mixture of genes. This 4-chromosomal organism is modeled below.
Procedure
1.
2.
3.
4.
Cut out the numbered boxes on the handout.
Place all numbers into the Ziploc bag.
Shake the Ziploc bag.
With eyes closed, take out one number. This represents which combination of gamete your
organism is providing.
5. Pair with a neighboring group and see what number they extracted. Write your zygote
combination below:
Gamete #1: __________
Gamete #2: ___________
6. Place the number back in the bag and shake the bag.
7. Repeat steps #4 and #5 an additional 3 more times, shaking the bag between each time. Write
your combinations below:
Gamete #1: __________
Gamete #2: ___________
Gamete #1: __________
Gamete #2: ___________
Gamete #1: __________
Gamete #2: ___________
Analysis
1. Out of the 4 times that you “created” a zygote, did you have any zygotes that were exactly the
same? Why or why not?
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Application
1. Humans have 23 chromosomes. How many possible daughter cell combinations are possible at
the end of meiosis?
2. Errors during meiosis occasionally occur. What would be the result of meiosis if a pair of
homologous chromosomes did not line up exactly during a crossing-over event?
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Answer Key
Analysis
Independent assortment
1. 16
2. 2n, where n=number of chromosomes
Crossing-over
1. Crossing-over creates recombinant chromosomes, chromosomes with a mixture of maternal and
paternal genes. When these genes go on to assort independently, it only increases the number
of combinations possible from meiosis.
Random fertilization
1. The different varieties of zygotes would be 16 x 16 = 256, so unless the students did not mix
their bags well, there should not have been any duplications.
Application
1. 223 = 8.4 million
2. The result would be a chromatid with missing genes (deletion) and a chromatid with extra
copies of genes (duplication).
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