Summer Research Program for Science Teachers

Adrienne Rubin

Bayard Rustin High School for the Humanities, Manhattan



How can we observe genetic variation within a species?


“Now, the Star-Belly Sneetches

Had bellies with stars

The Plain-Belly Sneetches

Had none upon thars.

Those stars weren’t so big.  They were really so small.

You might think such a thing wouldn’t matter at all.”

 Dr. Seuss

The Sneetches and Other Stories



When we think of biodiversity we often think of the many varieties of plants and animals one can find in the world.  Indeed, this idea struck Charles Darwin during his voyage on the HMS Beagle.  [9-12 Content Standard G- Historical perspectives] In a broader concept, biodiversity can mean the variation one can observe within a species.  This is called intra-species diversity. 

In this activity, you will observe intra-species variation, specifically two different strains of the same bacteria.  You will see how the interactions of these two varieties will affect change in their offspring while the original strains remain unchanged.



1.     Define intra-species variation. [9-12 Content Standard C- Biological evolution]

2.     Perform standard microbial streaking procedures.

3.     Collect data.

4.     Draw observations.



1.     Bunsen burner

2.     Bacterial Synergism Demonstration Kit (Carolina #AA-15-4744)

3.     Disinfectant

4.     Inoculating loop

5.     Serratia marcescens WCF (colorless mutant strain)

6.     Serratia marcescens 933 (colorless mutant strain)

7.     Synergism agar (5g peptone, 10ml glycerol, 20g agar in 1L distilled water)

8.     Sterile petri dishes (3 per group)

9.     Parafilm

10. Wax pencils

[Teaching Standard D- Make accessible science materials]


1.     Give students 3 petri dishes with sterile synergism agar.

2.     Label one petri dish “strain WCF” as a control.

3.     Label the second petri dish “933 strain” as a control.

4.     With a wax pencil, draw a line on the third petri dish as to divide it in half.

5.     Label one side WCF and the other side 933.  This will be the experimental plate.

6.     Streak one half of the first dish with the WCF culture of the bacterium Serratia marcescens.  Streak one half of the second plate with the 933 culture of Serriatia.  See diagram below.


                   WCF (c)                                                     933 (c)                


7.     In the experimental plate, streak both bacterial cultures on their appropriate sides, being careful to streak the two bacteria within one centimeter of each other at their closest point.  See diagram below.



                                                   WCF              933


8.     Seal each inoculated dish with a strip of Parafilm.

9.     Incubate these plates right side up (cover on top) at room temperature, and observe each plate for three to four days.

10. Draw your observations below.


              WCF-c                                        933-c                               WCF  933


11. After three days, take a small sample from each of the bacteria on the experimental plate.  Streak them back to the appropriate control plates. i.e. using proper bacterial methods, streak experimental WCF on the unused half of the control WCF plate.

12. Repeat step 11 for the 933 control and experimental plate.

13. Reseal each control plate with parafilm and incubate.

14. Draw your observations below.







                    WCF – c                                                          933 – c




1.     Why must an inoculating loop be heated in the Bunsen burner before and after inoculation? _______________________________________________________


2.     Compare the control plates to the experimental plates after three days.  Describe what you see. 



3.     What is your hypothesis as to what happened? ______________________________________________________________________

[Teaching Standard B- Orchestrate scientific discourse]


4.     After the seven days of incubation, describe what has happened in each of the control plates. _______________________________________________________________


5.     Does the change that occurred in the experimental plate a genetic change or an environmental change? Explain. 


[Content Standard Unifying Concepts- Evidence and explanation]

  6.     Design an experiment that would test your hypothesis about genetic vs. environmental changes. 


[9-12 Content Standard A- Design a scientific investigation]


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