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Name _____________________________ Date ___________ Period_______
Lab: Modeling Crossing Over
Adapted from Glencoe’s Biology: The Dynamics of Life
Problem:
How does crossing over affect the genes passed down to the daughter cells?
Hypothesis: _________________________________________________________
__________________________________________________________________
Experiment:
Materials:
Modeling clay
2 twist ties
Procedure:
1. Roll out four long strands of clay (of 2 different
colors of clay) at least 10 cm long to represent
two chromosomes, each with two chromatids.
2. Label the chromatids with the allele letters seen
in the pictures to the right. These alleles code
for 3 different traits. What is the genotype of
this person for each trait?
______________________
3. Use the figure to the right as a guide in joining
and labeling these model chromatids. Although there are four chromatids,
assume that they started out as a single pair of homologous chromosomes prior
to replication. (1 from the person’s father, 1 from their mother) The figure
shows a tetrad formation during prophase I of meiosis. When does replication
of chromosomes occur? ______________________
4. First, assume that no crossing over takes place. Model the appearance of the
four gamete cells that will result at the end of meiosis. Record your model’s
appearance by drawing the gametes’ chromosomes and their genes in the data
table.
5. Next, repeat steps 1-3, however, assume that
crossing over occurs in the last gene. See the
picture at the right as an example.
What does the twist tie represent? ________________
What does each color of clay represent?
_____________________________________________
What do the different letters represent?
__________________________________
___________________________________________________________________
Name _____________________________ Date ___________ Period_______
Data:
No Crossing Over
Appearance of Gamete Cells
Crossing Over
Appearance of Gamete Cells
Conclusions:
1. Define crossing over and explain when it occurs.
2. Compare any differences in the appearance of genes on chromosomes in gamete
cells when crossing over occurs and when it does not occur.
3. Crossing over has been compared to “shuffling the deck” in cards. Explain what
this means.
4. What would be accomplished if crossing over occurred between sister chromatids?
Explain your answer.