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.
Gamete Formation Gamete formation results from meiosis, which is split into two stages: meiosis I and meiosis II. Meiosis I Function: Separate homologous chromosomes Result: Haploid cells with duplicated chromosomes Diploid cell with duplicated chromosomes Haploid cell with duplicated chromosomes Meiosis II 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:
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
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.
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. Cut out the numbered boxes on the handout. 2. Place all numbers into the Ziploc bag. 3. Shake the Ziploc bag. 4. 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?
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?
Answer Key Analysis Independent assortment 1. 16 2. 2 n, 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. 2 23 = 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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