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Phil Talbot and Vanessa Bishop
The main objective of this activity is to illustrate
the variation that results from crossing-over during prophase I of meiosis.
Other sources of variation from generation to generation are: (1) independent
assortment (223 possible gametes that could be formed
in humans), and (2) random gamete pairing. This exercise, however,
looks mainly at the results of crossing over by using different colors
to represent different sets of chromosomes.
At the end of the activity, you should be able to
see the possible genetic recombination that results from crossing over
after 3 generations which will end up with an interesting combination of
blue, green, pink, and beige.
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Using a karyotype template that has alleles marked on each of
the 46 chromosomes, cut out the 23 pairs of chromosomes for each grandparent,
the first generation. Place the chromosomes in their respective
circles on the poster board, representing the genome of the grandparents.
Each grandparent cell should contain 46 (23 pairs) of chromosomes; each
individual set should be composed of two shades of the same color. Explain
why the homologs are of two different shades.
Do this for both sets of grandparents. This first
row of circles on the poster board represents the genome of each of the
grandparents (blue and green for the two males and pink and beige) for
the two females). Since in the next step you will be using the chromosomes
of the grandparents, be sure to write the grandparents' genotype in their
respective circle.
As the cells of this generation
form gametes, they go through crossing-over during meiosis. To represent
crossing-over that occurs in prophase I of meiosis you will randomly
cut both ends of each of the chromosomes (shades of blue) at the same loci
(you choose where to cut). Tape the ends onto their homologs. You should
now have 23 pairs of chromosomes that are each combinations of two
shades of the same color. For the female, you can cross over the two X's,
but for the males this will not be possible. The "X" and "Y" chromosome
should not cross-over. You may think that crossing over at two places on
a chromosome is too much, but in fact, in humans, there is an average of
three cross overs per homologous pair. Remember that many different chromosomal
combinations of gametes are possible due to independent assortment. However,
from each primary sex cell only four gametes are produced in males, and
only one gamete is formed in females. To complete the process of meiosis,
randomly select a set of homologous chromosomes and move them
into one of the gametes. Move the other haploid set into the second gamete
of the grandparent. Do the same for the other three grandparents.
Notice that the gametes are made up of chromosomes of different shades
of the same color. To mark the potential variation in chromosomal
combinations, observe the gametic genotypes other teams selected. Remember
that all teams started with identical chromosomes.
Since you want to remember the original alleles
of the grandparents, write them on the respective circle on the poster
board. Unless, of course, you are willing to cut as many chromosomes as
you have individuals.
To represent fertilization for the
next generation, at random, select one gamete from the blue and
pink grandparents and place them in the large circle that corresponds
to one of the parents. Do the same for the other parent, making sure that
it is of the opposite sex. The parent circle should contain at this point
one diploid set of blue and pink together, and one diploid set of beige
and green together. This represents the parents' genome. Do not forget
to write out the alleles for the gametes you have chosen (use colored pencils
that correspond to the chromosome colors). Again, go through the process
of crossing-over by cutting the ends of the chromosomes and exchange
them with the parent's other homolog (blue crossed with pink and green
crossed with beige). Place the chromosomes back in the parent circles.
Randomly select a haploid set of chromosomes from the one parent and place
them in one gamete and the remaining set in the second gamete. Do the same
for the other parent. This represents the genome of the parent's gametes.
One more time, at random, select a gamete
from each parent and place the chromosomes into the circle representing
the child. This represents the child's genome. Write out the child's genotype
in the respective cell. Randomly cut off the ends of homologous chromosomes,
switch ends with the homolog, and tape together. Return the chromosomes
to the child's circle. You should now have 23 pairs of chromosomes in the
child's cell. Each chromosome should be composed of four colors with combinations
of shades of each color (if the baby is a male, the Y should be a solid
color).
Now, randomly select a haploid set from the child's
genome and place it in the one gamete. Place the other haploid set in the
other gamete. Compare your child's genome (color pattern and alleles) to
that of the rest of the class. Your teacher may instruct you to make a
comparison with other classes. Construct a data table in order to organize
your information.
Layout of the board (one
per team)
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Phil Talbot and Vanessa Bishop to
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