Genetics of Wisconsin Fast Plants
Dihybrid Cross
Overview:
In this lab, you will investigate the inheritance of two traits in fast plants. Stems can
either be purple or anthocyaninless (nonpurple) as seen in the picture below. Cotyledons (first
leaves) can either be green or yellow. The parental generation was a cross between true-breeding
purple stemmed, green leaved plants with true-breeding yellow stemmed, yellow leaved plants.
You will plant and self-pollinate the F1 generation. You will harvest the seeds, plant the F2
generation and score the resulting progeny. In addition, you will perform a back-cross of the F1
generation to the recessive parental type (called a test-cross). Again, you will harvest and plant
the seeds and score the resulting progeny. Finally, you will perform statistical analysis on the
data to test your hypotheses.
1. Based on the information presented
above, write a hypothesis describing
the expected outcome of both the
self-pollination of the F1 generation
as well as the back-cross. Write both
of these hypotheses in your lab
notebook.
2. Plant F1 seeds as below:
a. Obtain a Styrofoam quad.
b.Insert a wick into the bottom of
each container.
c. Fill each container half way with
potting soil.
d.Place 4 yellow osmocote fertilizer
pellets in the soil of each container.
e. Fill each container nearly to the top with soil.
f. Moisten the soil thoroughly (the wick should drip)
g.Sow 3 seeds in each container.
h.Cover seeds with a very small amount of soil. Planting too deeply will prevent growth.
i. Fill the reservoir in your growth pod, soak the wicking strip and circle, and assemble the
growth pod.
j. Place an anti-algal square in the reservoir of the growth pod.
k.Place your containers on the growth pod, ensuring that the wick on the bottom of each
container contacts the moist wicking circle.
l. Place your growth pod on the light stand, so that the tops of your containers are
approximately 15 cm from the lights.
3. Care for your plants.
a. Water from the top every day for the first 5 days.
b.Check the level of water in the reservoir each day. Providing sufficient water is necessary
for plant growth.
4. Two to four days after planting your F1’s, use the same procedure to plant a quad of
recessive parental plants.
5. When your plants have flowered, self-pollinate using a bee stick made from a toothpick and a
bee thorax (middle section). Make sure to go from plant to plant because fast plants will not
self-pollinate. When you have finished self-pollinating, use the same bee stick to transfer F1
pollen to a single individual of a recessive parental type. Discard this bee stick since it is
now contaminated with recessive pollen.
6. After 5 days of pollination, remove all unopened buds. This will force the plants to put their
energy into seed growth.
7. Twenty days after the last germination, remove the plants from water and allow to dry for 5
days.
8. Harvest seeds by gently breaking the pods apart over a piece of white paper. Collect the
seeds for planting.
9. Take the mass of your seeds.
10. Plant 16 of your harvested seeds in your Styrofoam quad (you will need to clean it first) as
described before except that you will not use fertilizer pellets. If you use fertilizer pellets, it
will be difficult to see the purple stem phenotype.
11. After 5 days, score the phenotypes of the plants. You should have four different phenotypes.
One trait is purple stem/non purple stem (below
left). The other trait is yellow/green leaf (at
right).
12. Compile data across both AP Biology classes.
13. Conduct χ2 analysis of your data, and of the class data.
14. Discuss the meaning of your data.
15. Map out a breeding plan to recover the parental types from your F2’s. Describe what crosses
you would need to do to demonstrate that you have recovered the parental types. For each
cross, describe the results you expect.