genetics - National Science Teachers Association

CORN SNAKE
GENETICS
Students learn about the fundamentals
of Mendelism by studying corn snakes
Kristin King
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n an attempt to generate student enthusiasm on the
subject of genetics, I developed a Punnett square activity centered on the genetics of corn snakes to teach
students about Mendelism and genetic diversity
(Figure 1). As we began the activity, however, some unexpected twists occurred that allowed for investigation into
corn snake anatomy and behavior. Overall, this turn of
events enhanced the study of genetics and provided student opportunities for independent research, laboratory
investigations, and long-range data collection.
Engaging students
While researching textbooks and reminiscing about my
own college days working with fruit flies, I contemplated
many ways to teach genetics to my students. The study of
pea plants, although a necessary component of teaching
genetics, did not seem exciting enough to captivate my
freshmen students. In search of something more engaging, I considered teaching genetic traits using my
bearded dragon, Pagona vitticeps. I ran the idea past a
local pet store owner who suggested corn snakes as a
better fit for studying varied traits in the classroom. We
discussed the phenotypes of heterozygous and homozygous dominant and recessive traits in various specimens
of corn snakes. He indicated that varied morphs are becoming more common as herpetologists practice selective
breeding. I thought about this idea for a month or so
before I took action and went wild with the concept.
Live specimens
Keyword: Punnett squares
at www.scilinks.org
Enter code: TST010403
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When I first introduced the idea
of studying Mendelism through
corn snake genetics, we had no
live corn snake specimens in the
classroom. Therefore, as a supple-
T h e S c i e n c e Te a c h e r
ment to the activity in Figure 1, I provided a PowerPoint
presentation (available online at mrskingsbioweb.com/
Mendelism_files/frame.htm) with visual images of different
corn snake phenotypes along with a deck of identification
cards labeled with snake phenotypes and trade names
(Figure 2, p. 52). Before beginning the activity, however,
something unexpected occurred.
The morning the lab was scheduled to begin, a group of
students brought an injured wild type (Normal) corn snake
into the classroom, which had apparently been run over by a
vehicle. After great attempts to revive this snake, it died
leaving us with a unique opportunity—preservation. I postponed the onset of the activity to take advantage of the
opportunity at hand and to teach students about anatomy.
The following morning, I, along with another teacher
who is an expert in skinning snakes, removed the snakeskin and made an incision around the neck. We both
used gloves as a safety precaution. While gently pulling
back the snakeskin we noticed the trachea was visible for
5 cm to 6 cm. We thought it was exposed due to the
snake’s accidental demise; however, we later found out
this was not the case. We had difficulty removing the
skin at first, but after we passed the damaged region it
peeled back with ease. We then placed the remains on a
large porcelain dissecting tray and dried the carcass in
the hood in my classroom.
Later in the day as class began I held up the
snakeskin for my students to view where the actual damage had taken place. (Again, I wore
gloves and did not allow students to touch the skin.)
Bloodstains embedded in the skin, ranging approximately 16 cm to 27 cm, appeared in regions along the
snake’s body. We discussed how the anatomy of the
snake differed from other creatures. For instance, while
dissecting the snake it became clear that the flesh on the
FIGURE 1
Corn snake genetics activity.
Students become novice corn snake breeders and try their newfound knowledge at developing their own corn snake stock.
Equipped with knowledge of monohybrid and dihybrid crosses, corn snake identification cards, and a morphology tracking chart
(available online at mrskingsbioweb.com/Mendelism_files/frame.htm), each lab group embarks on the hidden world of heredity.
Directions
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Use the cards provided to select your mating pair.
Determine and record the genotype, phenotype, and trade name of each snake, P1.
Make a Punnett square to show what your possible outcomes (ratios) will be for all your offspring, F1.
What might you expect in the F2 generation? Pick any two genotypes from the F1 square and breed them.
What are the gamete combinations? State whether each snake is homozygous dominant, heterozygous dominant,
or homozygous recessive in all matings.
Refer to the Punnett square matrix below and label each snake using trade names and including proper prefixes:
hyper-, hypo-, a-, and an-.
Method
Step 1. Choose two snakes using the deck of
corn snake cards. Determine the genotype of
both sexes using the rubric below. Make a
dyhibrid cross of the P1 using algebra skills to
determine phenotypic ratios of F1.
Male: Anerythristic, Bbrr.
Male: Br Br br br
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Female: BR BR bR bR
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Male: BR Br bR br
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Female: bR bR bR br
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Repeat the process for the Punnett square.
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Determine the phenotypic ratios for F2 and compare them to the corn snake cards.
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Female: Normal, BbRR.
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Step 3. BbRr × bbRr
BbRR, 3 (Normal); BbRr, 3 (Normal); Bbrr, 2 (Anerythristic);
bbRR, 3 (Amelanistic); bbRr, 4 (Amelanistic); bbrr, 1 (Snow).
Punnett square matrix
Genotype
Phenotype
Trade name
Br
Br
br
br
BBRR
Black, Brown, Red, and Orange
Normal (wild type)
BR
BBRr
BBRr
BbRr
BbRr
BbRR
Black, Brown, Red, and Orange
Normal (Hypo ?)
BR
BBRr
BBRr
BbRr
BbRr
BBRr
Black, Brown, Red, and Orange
Normal (Hyper ?)
bR
BbRr
BbRr
bbRr
bbRr
BbRr
Black, Brown, Red, and Orange
Normal
bR
BbRr
BbRr
bbRr
bbRr
BBrr
Black or gray, occasionally with yellow
Anerythristic
Bbrr
Black or gray, occasionally with yellow
Anerythristic
bbRR
Red and Orange
Amelanistic
bbRr
Red and Orange
Amelanistic
bbrr
White, occasionally with yellow
Snow
Step 2. From this acquired information,
determine the phenotypes of the F 1
generation. Choose two snakes from the F1 to
breed and duplicate the same process as Step
1 to conclude the F2 generation results.
ventral side was disconnected all the way from the
esophagus to the end of the tail. After thinking about this
phenomenon from dissection day we realized this is how
snakes are able to eat and swallow such large prey whole.
This unique feature allows a snake to expand its body’s
width without tearing its dermis. For visual effect I
showed the class a photograph from the Encyclopedia of
Animals (Cogger 2002) of an African rock python swallowing an impala. The class could not believe a snake
could eat something so large. We discussed not only the
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FIGURE 2
PHOTOS COURTESY OF THE AUTHOR
Samples of snake identification cards.
Amelanistic and
Anerythristic
Albino Greenish Snow
Hypomelanistic and
Anerythristic Type A
Ghost
should always be fed in their tanks, never
in the open.
The new snake has been a wonderful addition to our classroom and an integral part
of our studies in corn snake behaviors, eating habits, benefits to pest management, and
their interactions with the environment.
After our little detour, we made our way
back to studying Mendelism. With all of their
newly acquired knowledge on snakes,
students conducted the activity in Figure 1
(p. 51) and learned how to set up Punnett
squares and interpret the phenotypic ratios to
determine genetic diversity in corn snakes.
The activity
I began the activity by discussing Gregor
Mendel and his famous pea plants. In addition, we reviewed the meaning of the
terms dominance, recessiveness, segregation,
and independent assortment and then applied this knowledge to coloration in corn
snakes. The class viewed the PowerPoint
presentation “Mendelism Through Corn
Snake Genetics,” which includes genetic
terminology, step-by-step instructions for
calculating Punnett squares, and photographs of various genotypes and phenotypes of corn snakes (Elaphe guttata). StuHypomelanistic
dents also studied the deck of 27
Albino Amelanistic
Normal
identification cards labeled with snake
Heterozygous for Snow
phenotypes and trade names (Figure 2).
When the lab was complete, each group
had their P1 gametes from the cards they
chose, an F1 Punnett square with genoway the snakeskin stretches but also how the jaw of the
types, phenotypes, and trade names, and an F2 Punnett
snake dislocates during feeding and then snaps back into
square with genotypes, phenotypes, and trade names. If
place once the muscles have forced prey down toward
for some reason students came up with a morph that had
the snake’s stomach.
not been discovered, they had the privilege of naming it.
Several days later one of my students found a beautiful
The twist in the lesson was for students to keep in mind
Hypomelanistic female corn snake in his back yard, which
that iridophore cells in a corn snake’s skin are like wild
measured 93 cm in length. We waited several days for the
cards that combine with the snake’s three basic pigments
snake to become accustomed to its new home before feed(melanin/black, erythrin/red, and xanthin/yellow), which
ing it for the first time in captivity. Many times wild corn
create stunning morphs. This phenomenon causes unexsnakes will not eat in captivity and must be force-fed, but
pected coloration differences like white, blue, green, rainthis snake ate its first small mouse without hesitation. As a
bow, and other unique combinations that change the basic
safety precaution, I fed the snake instead of letting
pigments of the corn snake’s skin. This information
my students feed it. Snakes become active and go
caused confusion within the groups. For that reason we
on the hunt in the evening around dusk. I usually
stuck with the basic color combinations listed in the
go into my classroom on a Friday evening and feed
Punnett square matrix (Figure 1, p. 51).
the snake once a week. If snakes feel threatened (i.e., viStudents were surprised to discover the variations that
brations from students banging around the lab stations)
occurred in the F2 generation of their snakes. Many students
asked for help setting up the Punnett squares and determinthey will regurgitate their food. To eliminate this factor I
ing the correct phenotypes. After one class period students
feed the snake in the evening, allowing the snake a chance
seemed to have the concept of genetic crosses mastered and
to digest its food before the “vibrations” occur. Snakes
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T h e S c i e n c e Te a c h e r
PHOTOS COURTESY OF THE AUTHOR
Students study the snake identification cards.
wanted to try breeding another set of snakes. Students could
not believe all the possibilities and were eager to breed real
snakes (only through actually breeding these snakes would
we be able to determine what recessive traits are being
masked). In this lab, students learned the fundamentals of
setting up Punnett squares and how to interpret the phenotypic ratios from the data collected to determine genetic
diversity in corn snakes. We discussed what the students
might want to know about a breeding pair and offspring if
they were corn snake breeders. We also discussed how these
results might affect the buying habits of the consumer.
The genetics activity expanded into lessons on active
and passive immunity and how knowledge of venomous
and nonvenomous snakes contributes to medical practices. Our discussions also involved careers in corn snake
breeding and genetics. We discussed reptile (snake)
anatomy, morphology, and taxonomy. We were able to
incorporate a lab on endothermic/exothermic reactions
in the ecosystem and how it applies to warm-blooded
and cold-blooded animals as well as discuss where snakes
lie in the hierarchy of the food chain.
My class participated in the entire scope of the corn
snake study (both the planned and unexpected aspects)
with enthusiasm, and most students thoroughly grasped
the concept of Mendelism, Punnett squares, and genetic
diversity. Students continue to come into class before and
after school just to talk about snakes. Occasionally, I see
students in the local pet store, which I visit on a frequent
basis, looking at reptiles. Some students have expressed
interest in careers as biologists and herpetologists as a result of this project. What a wonderful beginning!
As a result of this successful study, I purchased three
more corn snakes with different phenotypes to add to our
classroom: Albino Anerytheristic Ghost (male), Albino
Amelanistic Heterozygous for Snow (male), and Greenish
Snow Corn (female). My ambition is to have future science
students predict the F1 and F2 generations using Punnett
squares, observe the F1 and F2 generations through selective breeding, track and graph their growth rates, and
calculate mass differences over time. Snakes in Florida
only breed once or twice a year, therefore this process will
take some time to determine the F2 in the bloodlines. The
freshmen I taught in this first year of corn snake genetics
will most likely not see the results until they are juniors.
The ongoing research will be quite interesting and will
give many students the opportunity to participate as they
check and recheck each other’s data over time.
I am excited about the development of reptiles in my
classroom and the teaching of Mendelism through corn
snake genetics. I have noticed many positive results in
my students’ grades, not to mention many engaged, smiling freshmen faces!n
Kristin King is a teacher at Eau Gallie High School,
1400 Commodore Boulevard, Melbourne, FL 32935; email: [email protected].
References
Cogger, H. 2002. Encyclopedia of Animals. New York: Barnes and
Noble Books.
National Research Council (NRC). 1996. National Science Education Standards. Washington, D.C.: National Academy Press.
On the Web
Connecting to the Standards
The corn snakes genetics activity addresses the following
National Science Education Standards for grades 9–12.
NSES Teaching Standards A and B (NRC 1996, pp. 30, 32);
NSES 9–12 Content Standards A, B, C, E, F, and G (NRC
1996, pp. 173–204);
N NSES Program Standards B and D. (NRC 1996, pp. 212, 218).
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Corn Snake Genetics Primer: www.pitt.edu/~mcs2/herp/genetics.html
Corn Snake Morphs: www.cornsnakemorphs.com/genetics.html
Florida Museum of Natural History: www.flmnh.ufl.edu/natsci/herpetology/fl-guide/Elaphegguttata.htm
Reptimania: www.reptimania.co.uk/genetics.htm
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NSTA Connection
Visit www.nsta.org/159&psid=2 for NSTA’s position statement
“Guidelines for Responsible Use of Animals in the Classroom.”
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