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Tuesday, April 17, 2012

The Importance of Environmental Influences on Gene Expression



All students know that their hair color and their eye color is a result of the alleles that they have inherited from their parents.  But do your students know that the environment plays a role in the expression of these alleles?

The phenotype of an individual often depends on the influences from the environment.  A snowshoe rabbit has white fur in the winter months and brownish fur in the summer months.  Why?  If the rabbit is white in the winter, it will be camouflaged  and will have a greater chance of survival.  The same is true of a brown rabbit in the summer months.  A particular allele codes for coat color in these rabbits.  During the winter, the snowshoe rabbit will have white fur because these pigment producing genes do not function in cold weather.  When the weather warms in the spring and summer, the genes function to produce pigments and the coat becomes brown.

The million dollar question is this:  Is heredity or environment more important in determining the kinds of traits that appear in the offspring?

I recently had my students explore this very question in a laboratory activity.  Students were given 10 corn seeds to plant.  Half of the students were asked to leave their container of planted seeds near a window in our lab.  The other half of the class was asked to place their container of corn seeds in complete  darkness.    The seeds were watered as needed and left to grow.

The trait the students were asked to observe was the color of the stem and leaves.  Did the seeds grow into plants with green stems or into plants that were albino?  Stems are green due to the production of chlorophyll.  Chlorophyll production is controlled by a dominant allele.  The absence of a dominant allele results in a plant that is albino.

The students were given seeds that were the offspring of heterozygous parents.  Therefore, it is expected that 3/4ths of the seeds will grow into corn plants that are green, and 1/4th of the seeds will grow into plants that are white or albino.

After allowing the seeds to grow into seedlings, students return to the lab to count their offspring.




In the above photos, both trays of corn were grown in the light.  As you can see, some of the plants have green stems, while other plants have albino stems.  Class data was tabulated and we were very close to the expected 3:1 ratio of green to albino stems.

Take a look at the plants in the photo to the right.  The plants on the right side of the photo were grown in complete darkness.  All of these plants are termed albino.  The expected 3:1 ratio was not supported.  100% of the plants grew into albino plants.  What is the difference between the albino plants grown in the light and the albinos grown in the dark?   The albino plants that were grown in the light are albino due to their genetics.  These plants have two recessive alleles for chlorophyll production.  Without the dominant allele, chlorophyll production is not possible.  The albino plants that were grown in complete darkness are albino because of their environment.  Some of these plants may have the dominant allele, but in the absence of sunlight, the dominant allele is not expressed.  

To make this argument just a bit stronger, all students were asked to let their seedlings continue to grow for another 24 hours, and all trays of seedlings were left in the light.  Plants that were initially grown in the dark, were left in the light for 24 hours.  At the second observation, 3/4ths of these albinos had developed chlorophyll and were green.

So back to the original question...... Is heredity or the environment more important in gene expression?  Every student in my class now knows that having the dominant allele is not always enough for the trait to be expressed.  Sometimes the dominant allele and the proper environmental condition work together in the expression of the trait.

Give this lab a try.  Your students will love it!
Happy Teaching!


PS - I am proud to display the 5-Star Blogger award.  Check out the Five-Star Blogger Challenge posted on The Organized Classroom Blog to see some awesome Five-Star Blogs!


2 comments:

  1. Great post - definitely a 5-Star Blogger! Thanks for linking up!

    Charity
    The Organized Classroom Blog

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  2. Stopping over to visit my fellow 5-Star Bloggers! This is such a neat experiment. We are studying the plant kingdom in our homeschool right now, so I am going to plan to do this soon! I did a similar experiment as my 6th grade science fair project, but my "dark" plants didn't grow.

    www.theprincessandthetot.blogspot.com
    www.bentosonthebayou.blogspot.com

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