1) A science project explores advanced concepts in Mendelian genetics through genetically engineered carnivorous "plantfairies" and their dangerous mutant offspring, "plantmonsters". 2) Students perform dihybrid crosses to identify genotypes of plantfairies and determine which may carry alleles for the plantmonster mutation. 3) The results of the crosses help researchers decide which plantfairies can be safely kept and which may need to be destroyed to prevent the spread of the dangerous mutation.

1. #Title: Plantfairies and plantmonsters: adventures in Mendelian genetics
#Targeted Level: 11th
#Difficulty: Advanced 2
#Time Required: 1 hour
#Notes on Time Required:
#Prerequisites: You need to be familiar with concepts and vocabulary introduced in the
science project called “The Odds of Being You: a zoomonster from planet Planktonia”. You also
need to understand the concepts of DNA, chromosomes, and genes.
#Material Availability: Readily available
#Material Availability Notes: All the materials for this lab can be found at home.
#Cost: Very low
#Adult Supervision Required: No
#Safety:
#Abstract: In this science project, you are going to explore advanced concepts in Mendelian genetics.
Our "research scientists" have genetically engineered a plant, called "plantfairies", to control insect pests.
But something went wrong with our experiment, resulting in a mutant form (called plantmonsters) that are
dangerous to humans. You are going to help our research team by identifying the genotypes of the
plantfairies and their offspring in order to determine which plants may be carrying the dangerous alleles.
#Objective: To gain an understanding of advanced concepts in Mendelian genetics and make
predictions using dihybrid crosses.
#Hypothesis:
#Credits:
Jessica Watson, B.S. Ecology & Evolutionary Biology
#Background:
Gregor Mendel was an Austrian monk in the 19th century who uncovered many of the basic principles of
inheritance and genetics by his experiments with pea plants. By observing several generations of the
plant, he could determine which traits were dominant and which were recessive, as well as identify
several patterns of inheritance. He mated different varieties of plants, using a brush to transfer pollen.
Mating different varieties, or crossing different genotypes, of a plant is called hybridization.
Mendel started his experiments with purebred plants, that is, identical plants that when mated had
offspring with traits identical to their parents (such as having all purple flowers). Purebreds are

2. homozygous for the trait that they exhibit, such as flower color. They may be homozygous dominant
(PP=purple flowers) or homozygous recessive (pp=white flowers). When Mendel crossed purebred
purple plants with purebred white plants, all plants in the first generation of offspring were purple. The
punnett square for that cross is as follows, using the letter p to represent alleles for flower color:
Alleles from the purebred purple
plant
P P
Allelesfromthe
purebredwhite
plant
p
Pp Pp
p Pp Pp
Table 1. Punnett square for monohybrid cross of two purebred plants.
When breeding experiments only follow one trait, such as flower color. They are called monohybrid
crosses. As you can see from the punnett square above, all offspring produced were heterozygous for
flower color. Because they had one dominant allele (P) in their genotype, they all exhibited the dominant
purple phenotype. However, when Mendel crossed the heterozygous offspring of two different purebreds,
he observed a different pattern of inheritance in this second generation of offspring.
Alleles from heterozygous plant 1
P p
Allelesfrom
heterozygousplant
2
P
PP Pp
p Pp pp
Table 2. Punnett square for monohybrid cross of two heterozygous parents.

3. The second generation of offspring had the following genotypes: PP, Pp, Pp and pp. Instead of having
all purple flowers, some offspring had white flowers. Those with a dominant allele (P) had purple flowers,
while those with two recessive alleles (pp) had white flowers.
A ratio is a mathematical expression to describe the proportion of different items in a set. If there are 3
boys and 2 girls in a room, the ratio of boys to girls in the room is 3:2. Similarly, the ratio of purple to
white flowers, or phenotypes, in the offspring of the heterozygous cross is 3:1. The ratio of genotypes in
the offspring is slightly different. Since there were 3 different genotypes in the offspring (1 PP, 2 Pp and 1
pp), the ratio of genotypes is 1:2:1.
Mendel’s breeding experiments became more complicated when he performed crosses to observe more
than one trait at once, such as pea color and texture, called dihybrid crosses. Punnett squares for
dihybrid crosses are larger. The following will illustrate an example of the punnett square resulting from a
dihybrid cross of pea plants, with both parents being heterozygous for pea color (Yy) and pea texture
(Rr).
Dominant pea color is yellow (Y), while recessive color is green (y). Dominant pea texture is round (R),
while recessive is wrinkled (r). The genotype for both parent pea plants is YyRr. The first step to make
this punnett square is to determine the combinations of alleles that each parent can give separately. To
do this it’s necessary to use a technique from algebra called F.O.I.L. (first-outer-inner-last) on the
genotype YyRr.
The genotype for trait one (pea color) is Yy, while the genotype for trait two (pea texture) is Rr. First,
multiply the first letter of each trait’s genotype (Y*R), then the two outer letters (Y*r), then the two inner
letters (y*R) and finally the last letters (y*r). These are the four possible combinations of alleles that each
parent plant can donate to offspring. To determine the offspring, you cross multiply all four parent alleles.
As you can see from the punnett square below, a dihybrid cross results in many more possibilities for the
genotype and phenotype of the offspring.
Alleles from heterozygous plant 1
YR Yr yR yr
Allelesfromheterozygousplant
2
yr
YyRr Yyrr yyRr yyrr
yR YyRR YyRr yyRR yyRr
Yr YYRr YYrr YyRr Yyrr

4. YR YYRR YYRr YyRR YyRr
Table 3. Punnett square for dihybrid cross of two parents heterozygous for both traits.
#Terms and Concepts:
hybridization
ratio
mutation
monohybrid cross
dihybrid cross
testcross
#Questions:
1. Can you think of any examples of organisms that have undergone a mutation that can be harmful
to people?
2. What is a mutation at the genetic level?
3. What causes mutations?
#Bibliography:
#Materials and Equipment:
paper
colored pencils
#Experimental Procedure:
Our imaginary scientists from "Plants-R-Us, Inc." have been experimenting with genetic engineering of
plants to use in biocontrol projects. Their intention was to create a carnivorous plant, called "plantfairies",
similar to Venus Flytraps, pitcher plants and sundews, which get nutrients by consuming insect pests like
mosquitos.
Something terrible happened and a bizarre mutation appeared in the third generation of engineered
plants that allows them to grow giant flowers with sharp, dangerous teeth, and a preference for eating
human beings, - these mutant plants have been named "plantmonsters". The plantfairies are harmless
dwarf flowers with "tongues" that "flick", to catch insects.
All plantmonsters are going to be destroyed by specially trained members of our research team; however,
many plantfairies still remain. Some plantfairies are carriers of the dangerous plantmonster genes and
can pass the gene on to future generations. Millions of dollars have gone into this research and our
scientists want to know if any plantfairies can be salvaged for the purpose of mosquito control. It’s your
job to make predictions about the offspring of these mutant plants in order to prevent the world being
taken over by these plantmonsters!!

5. You will need the included tables of plantfairy and plantmonster genotypes and phenotypes. Remember,
plantfairies are dwarves with "frog tongues", while plantmonsters are giants with "dagger teeth". A dwarf
with dagger teeth is still dangerous because it eats small mammals and birds, and carries the alleles for
dagger teeth.
Trait Dominant
(homozygous)
Recessive
(homozygous)
Heterozygous
(Dd, Tt)
Body size dwarf giant dwarf
Mouthparts frog tongue dagger teeth frog tongue
Dominant Recessive
Trait Alleles Genotype Phenotype Genotype Phenotype
Body size D,d DD, Dd Dwarf dd Giant
Mouthpart T,t TT, Tt frog tongue tt Dagger teeth

6. 1) The scientists have begun breeding experiments to determine the genotypes of the remaining
plantfairies and identify any individuals who may be carrying the dangerous recessive alleles. They have
isolated plantfairy #156 for you, so you can perform a testcross with a purebred plantfairy in order to
determine its genotype. Purebred plantfairies have known genotypes that are homozygous dominant for
all traits, so they are "dwarf" flowers with "flicking" tongues, that are able to catch insects.
When you perform the testcross of a purebred plantfairy with plantfairy #156, all offspring are
homozygous dominant for all traits. Given these results, what genotype does plantfairy #156 have? The
results of the dihybrid testcross are shown in the punnett square below. Since you know the genotypes of
all offspring and the purebred, you work backwards to solve the genotype of plantfairy #156. Is plantfairy
#156 safe to keep in the population?
Alleles from Plantfairy #156
Alleles from
purebred
plantfairy
?? (DT) ?? (DT) ?? (DT) ?? (DT)
DT DDTT DDTT DDTT DDTT
DT DDTT DDTT DDTT DDTT
DT DDTT DDTT DDTT DDTT
DT DDTT DDTT DDTT DDTT
2) Next, scientists want to know what possible genotypes will result from a cross between a purebred
plantfairy and a plantfairy carrying the dangerous recessive alleles. They give you one purebred
plantfairy (DDTT) and one heterozygous plantfairy (DdTt) to cross. Use punnett squares to show the
possible genotypes that will result from such a cross and calculate the probability of getting each
genotype. Set up a punnett square for a dihybrid cross as demonstrated in the background section.
Remember to use F.O.I.L. to determine all combinations of alleles that each parent can pass on to
offspring. Draw pictures of the offspring if it helps you to determine the proportions or ratio of each
phenotype resulting from this cross.
Alleles from purebred plantfairy
DT DT DT DT
Allelesfromheterozygous
plantfairy
dT
DdTT
DdTT DdTT DdTT
dt DdTt
DdTt DdTt DdTt
Dt DDTt
DDTt
DDTt
DDTt

7. DT
DDTT DDTT DDTT DDTT
Offspring Phenotypes: Dwarf, Tongue: _________________
Dwarf, Teeth: _________________
Giant, Tongue: _________________
Giant, Teeth: __________________
3) Now the research team wants to know how many of the offspring can be saved that result from
crossing two heterozygous plantfairies (DdTt). Follow the instructions in step 2, but this time both parents
are heterozygous. Draw pictures of the offspring if it helps you to quantify how many of each type of
phenotype will result from this pairing.
Alleles from heterozygous plant 1
DT dt dT Dt
Allelesfromheterozygousplant2
DT
DDTT
DdTt DdTT DDTt
dt
DdTt
ddtt ddTt Ddtt
dT
DdTT
ddTt ddTT DdTt
Dt
DDTt Ddtt DdTt DDtt
Offspring Phenotypes: Dwarf, Tongue: _________________

8. Dwarf, Teeth: _________________
Giant, Tongue: _________________
Giant, Teeth: __________________
#Discussion: Which cross yields more harmless plantfairies, the cross between a purebred plantfairy
and heterozygous plantfairy (step 2) or between two heterozygous plantfairies (step 3)? Which
genotypes do you recommend for extermination and why?
#Make it Your Own: Design your own biocontrol breeding experiments that go terribly wrong.
Choose two traits in this organism that mutate into a form that’s dangerous to humans. Perform dihybrid
crosses as in steps 2 and 3 and analyze your results.
#Answers to Questions:
1. Can you think of any examples of organisms that undergo a mutation that can be harmful to
people?
Bacteria regularly mutate such that they become resistant to antibiotics. MRSA, also known as
methycillin resistant Staphylococcus aureus, is one such example of bacteria that is resistant to
treatment with antibiotics. Treating infections from resistant bacteria is one of the challenges that
hospitals face.
2. What is a mutation at the genetic level?
A mutation is a change in the DNA sequence that can be as simple as a change to a single
nucleotide, such as a substitution or loss, or a change to more than one nucleotide. It can affect both
coding and non-coding segments of the DNA.
3. What causes mutations?
Mutations can occur naturally in the process of replicating DNA for cell division. There are also
factors in the environment called mutagens that can cause mutation. Some examples of mutagens
are ultraviolet light, certain chemical compounds as well as viruses and bacteria.