Download Practical Activity: Modelling Crossing Over Materials: Method

Practical Activity: Modelling Crossing Over
Guiding Question:
How does crossing over affect the genes on a chromosome, and
variation in offspring?
Materials:
Modeling clay
2 twist ties
Method
1. Roll out four long strands of clay about 10 cm long.
2. Join them in pairs to represent two double stranded
chromosomes, each with two chromatids, just like in prophase I.
Use the diagram to the right as a guide
You now have a tetrad formed during prophase I of meiosis.
First, assuming that no crossing over takes place.
3. Model the appearance of the four gamete cells that will result
at the end of meiosis. Meiosis I will separate the chromosomes
and Meiosis II will separate the chromatids, making 4 daughter
cells.
Record the four gametes’ chromosomes and their genes in the data table.
Next, when crossing over takes place between genes B and C so that one pair of non-sister
chromatids exchange their copy of gene C.
Repeat steps 1-3, however, because crossing over occurs between genes B and A you will need to cut
and rejoin two pieces of two non-sister chromatids.
See the picture below as an example.
Record the four gametes’ chromosomes and their genes in the data table.
© David Faure, InThinking www.biology-inthinking.co.uk
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Practical Activity: Modelling Crossing Over
Results
No Crossing Over
Crossing Over
Appearance of Gamete Cells
Appearance of Gamete Cells
Questions:
1. Define crossing over and state when it occurs.
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2. Compare the number of different allele combinations on the chromosomes in the gamete
cells when crossing over occurs and when it does not occur.
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3. Crossing over is responsible for some of the variation seen in offspring produced by sexual
reproduction. Explain what this means.
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This activity was adapted from an idea in Glencoe’s Biology: The Dynamics of Life
© David Faure, InThinking www.biology-inthinking.co.uk
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