Cross-pollination Lab: Selectively breeding a better plant
Question: How does selective breeding produce organisms with more desirable traits?
Background information: In the wild, plants often reproduce and make seeds by
transferring pollen between two flower parts (from the anther to the stigma). This is usually
done by wind or by natural pollinating animals such as bees, butterflies, and hummingbirds. The
pollen contains half of the DNA needed to make a seed, and the stigma contains the other half.
If a plant receives pollen from a different plant, it has the chance to get new traits from the
different combinations of DNA. Those new traits might make the new plants able to grow
better. Since we want to produce the best plants possible for growing crops, we don’t leave the
pollinating to chance; we choose two plants with good traits and trade pollen between them by
hand. This must be done carefully without any interference from wind or natural pollinators.
In this lab, you will choose two plants based on their genotypes and simulate several
crosses. Your goal is to eventually produce a plant that has four optimal genes for root length,
leaf width, stem height, and number of flowers (you are trying to produce a plant that has all
four dominant traits). R= long roots, L= wide leaves, T= tall stem, and F= double flowering.
Materials needed: Plant Cross Sheets, set of gene flippers (make sure there are twelve of both
colors)
Procedures:
1. Divide the gene flippers by colors and make sure you have the following letter
combinations in both groups:
TT
RR
LL
FF
Tt
Rr
Ll
Ff
Tt
rr
ll
ff
2. Choose two parent plants from the genotype options listed below; these will be the
original plants that begin the cross-pollinating project. As a good geneticist, you will
want to make sure that you have a dominant gene for each trait somewhere in the
parents’ genes.
TtRrllff
TtrrLlff
TtrrllFf
ttRrLlff
ttRrllFf
ttrrLlFf
3. Write the genotypes you just chose in the top rectangular boxes on the Plant Cross
Sheet; the horizontal line between shows that they are being bred to each other (crosspollinated). See example on back for help.
4. For the left box, find the four gene flippers that have the matching trait combinations.
Do the same for the right box. These flippers represent the traits that each plant can
pass on to the baby plants with a 50:50 chance every time.
5. Now it’s time to flip genes! Flip the eight pieces that you found in step 4 above. Record
the genotype that resulted in the left oval. This is the first baby plant of that generation
(the ovals and diagonal lines represent offspring). See example below for help.
6. Repeat step 5 and write the resulting genotype in the right oval. This is the other baby
plant of that generation. This is the F1 generation. See example below for help.
7. Now you need to pick two of the four plants you would like to cross-pollinate in the next
step. Be sure to pick plants that will give you the best chances of dominate genes in the
offspring. Draw dotted lines from these two plants down to the next two rectangular
boxes and write their genotype combinations in the boxes. See example below for help.
8. Repeat steps 4-7 on the Plant Cross Sheet, continually choosing the best parent plants
to get the most dominant genes. You may choose any two plants from past crosses to
breed in the new crosses, and plants may be chosen many times if needed. You are
finished when you have produced a baby plant with four dominant traits (at least one
capital letter for each trait).
Example: The cross below is just to give you an idea of where to write the genotypes and
how to take genotypes down to the next level of breeding. Do not copy this example; it
uses genotypes that are not available in your lab!
ttrrLlff
ttRrllff
TtRrllFf
F1 generation
TtRrllFf
??????
ttrrLlFf
ttrrLlFf
F2 generation
??????
Plant Cross Sheet
Engineers:___________________________
#1
F1 generation
F2 generation
F3 generation
F4 generation
F5 generation
F6 generation
F7 generation
F8 generation
F9 generation
F10 generation