Download Creating a Gene Map - Southington Public Schools

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Gene Mapping
In prophase I of meiosis, homologous chromosomes pair up and tangle together to form a “tetrad”. In a
tetrad the two chromosomes undergo a process known as crossing over. Because of crossing over, genes on
the same chromosome can end up with different arrangements of alleles than they had before meiosis began.
Genes cross over in direct proportion to the distance they are away from each other. In other words, if two
genes cross over only 10% of the time, they are close together. If the cross over 90% of the time, they are far
apart. Geneticists have used this information to create gene maps. A gene map shows the relative locations
of known genes.
Creating a Gene Map
Creating a map is a logic puzzle, similar to a brainteaser. The percentage of crossing over is called the
recombination rate. This rate is taken to be the distance between two genes on a chromosome. If the
recombination rate is 13%, the genes are thirteen units apart. If the rate is 35%, then the genes are 35 units
apart.
Use a list of recombination rates to arrange the genes. If enough combinations are shown, then the pieces
(the distances between genes) will only fit together one way so that the distances are correct for all
combinations given.
Example
A chromosome contains four genes, A, B, C and D. The recombination rates are: C-D 25 units, A-B 12
units, B-D 20 units and A-C 17 units. It is not as easy as it looks. If you start in the order presented, your
map first looks like this:
C
D
|----------25-------------|
Then skip the A-B info because they don’t fit yet (there’s nothing to connect to).
Add the B (20 units past the D):
Go back to the A-B info and
add the A (12 units from the B):
C
D
B
|----------25-------------|---------20---------|
C
D
B
A
|----------25-------------|---------20---------|-----12-----|
Wow, this is easy. Now we check the A-C distance and find they’re supposed to be 17 units apart. Our map
has them 57 units apart…doh! Obviously, the total distance is too far, so the A and B chromosomes must be
in between the C and D. Go back to two steps and place the B 20 units to the left of D instead of adding it on
to the right. Where will A fit so it is 12 from B and 17 from C? The correct map is shown below:
C
B
A
D
|--5--|-----12-----|----8---|
Now you try it.
Use the information on the back of this sheet and map the genes for a chromosome of a fruit fly. To
successfully create a map, remember to use the two P’s…patience and a pencil. You will probably change
your mind about the order several times!
Create a gene map in the space below for the X chromosome of a fruit fly. There are 7 genes to map in total.
There is a gene for body color, hair, bristles, and 4 different eye color genes (ruby, garnet, vermilion and
white)—each is a separate location. Choose a letter to represent the trait and write a key for your letter
choices. Then place the letters on a line with the correct distances indicated similar to the example on the
front of this sheet. *Note: no two genes can be in the exact same location on a single chromosome.
Genes
Frequency of crossing-over
Yellow body; vermilion eyes
33%
Bobbed hair; vermilion eyes
33%
White eyes; garnet eyes
42.5%
Yellow body; white eyes
1.5%
Vermilion eyes; garnet eyes
11%
Bobbed hairs; singed bristles
45%
Ruby eyes; singed bristles
13.5%
Ruby eyes; garnet eyes
36.5%
Bobbed hairs; garnet eyes
22%
White eyes; ruby eyes
6%
Gene Map for the X Chromosome in Drosophila