Genotypes, Phenotypes, Genetics, Oh my!
SC.7.L.16.2 Determine the probabilities for genotype and phenotype combinations using Punnett Squares and
SC.912.L.15.15 Describe how mutation and genetic recombination increase genetic variation.
Introduce how some traits are inherited by Mendelian genetics
Predict genotype and phenotype using Punnett squares
What are Traits?
Traits are characteristics you inherit from your parents. Ask students to identify traits in the 3 individuals in the figure
below (eye color, skin color, height, freckles).
You should mention that not all traits are visible. A useful video can be found at
Have students determine what their trait (bitter or not) is for tasting PTC (phenylthiocarbamide). Tasting strips are
available at Amazon.com https://www.amazon.com/Nasco-PTC-Paper-Strips-Vial/dp/B001D7FF5E
To explore more about PTC tasting http://learn.genetics.utah.edu/content/basics/ptc/
A gene carries the information (DNA) that determines traits. In the field of genetics, we can predict some of the traits
a child will have if we know the parents’ genes. [For more information on genes & DNA,
Show the following video to introduce Mendelian genetics: https://www.youtube.com/watch?v=Mehz7tCxjSE
Review all of the definitions in the video before practicing Punnett squares:
1. Traits are characteristics you inherit from your parents.
2. A gene carries information (DNA) that determines your traits.
3. Alleles are different variations of a gene. There are 2 alleles for every gene.
4. The 2 alleles for a gene that represents a specific trait are called genotype.
Ex) The genotype Bb represents hair color
5. Homozygous: the alleles are the same for a particular trait
Ex) BB, bb
6. Heterozygous: the alleles are different for a particular trait
7. Phenotype is the observable characteristic of a specific trait resulting from its
Ex) Blonde hair is the phenotype observed determined by genotype Bb
8. A dominant trait “hides” the recessive trait in the phenotype of an individual. A capital letter represents
the dominant form in an allele.
9. A recessive trait describes a trait that is covered over (or dominated) by another form of that trait and
seems to disappear. A lowercase letter represents the recessive form in an allele.
The Punnett square is a way of working out the possible genotypes and phenotypes of offspring.
We use two letters to represent the genotype. A capital letter
represents the dominant allele and a lowercase letter is the
abbreviation for the recessive allele.
In the example on the left, B=dominant, blue and b= recessive
Have students practice the definitions using the Punnett square above.
1. What is the trait?
2. What are the alleles?
B and b
3. What are the 3 different genotypes of the offspring?
Bb, BB, bb
4. Which genotypes are homozygous?
BB and bb
5. Which genotypes are heterozygous?
6. What are the phenotypes of each offspring?
Bb=blue; BB=blue; bb=red
How to generate a Punnett square:
Walk students through constructing a Punnett square using the PTC tasting gene. This demonstrates to students that
by knowing the parents’ genotypes for the PTC tasting gene, we can predict their kids’ ability to taste PTC.
Ask students if the kids of these parents will be able to taste PTC.
Answer: Yes, since the all genotypes have the dominant allele “T”.
Phenotype=can taste PTC
1. Put one letter (allele) above/
next to each box. It does not
matter which parent’s genotype
goes on the top/side.
2. Fill in the boxes by copying the
column & row letters down and
Fill in the boxes by
copying the column &
row le ers down &
across into the empty
Practice filling in a Punnett square
Fill in the Punnett square below and use the results to answer the questions below.
1. What are the phenotypes of each kid?
2. Which kids will be able to taste PTC?
3. Which kids will NOT be able to taste PTC?
Phenotype=can taste PTC
Sometimes mutations can occur in the DNA of specific genes that changes the phenotype. Most mutations are not
bad but just different.
A useful video for mutations http://learn.genetics.utah.edu/content/basics/mutation/
Phenotype=can taste PTC
Can taste Can taste
CanNOT taste CanNOT taste
Use Punnett squares to design a unicorn suncatcher
In groups, students will be assigned genotypes of unicorn “parents” for various traits and create Punnett squares.
Possible traits and genotypes are listed below. The trait “4 or 2 legs” refers to unicorns standing on four or two legs
(https://www.amazon.com/Plastic-Unicorn-Suncatchers-24-Pack/dp/B01KU3AAA6). Students will put the resulting
individual squares into cups labeled with each trait. Students will pick a random square from each trait and use that
phenotype to design their suncatcher. An example of the parent traits and one pair of genotypes is shown below.
The Punnett squares for each trait for Group 1 parent unicorns can be found at the end of this document. You can
include a mutation square in the cup and indicate the phenotype.
1. With a partner, pick one piece of paper from the cup labeled “TRAITS”. Write down the trait you picked.
2. With your partner, create a Punnett square for the trait you selected. Use the piece of paper from your TA
and follow the directions on it.
Trait Dominant Allele Recessive Allele
4 or 2 legs L l
Body Color B= silver b= pink
Hoof Color H= blue h= purple
Tail Color T= purple t= yellow
Horn Color W= pink w= black
Mane Color M= green m= blue
Group 4 or 2 legs Body color Hoof color Tail color Horn color Mane color
1 (father) LL Bb Hh TT ww Mm
1 (mother) ll Bb HH TT Ww mm
3. Once your table has created all the Punnett squares and put each individual square in the appropriate trait
cup, pick one square from each trait cup and write down the genotype and phenotype below.
PUT THE PAPER BACK IN THE CUP SO THE NEXT PERSON CAN PICK!!!
4. Once you’ve finished selecting all traits, use the markers to create your unicorn suncatcher. Make sure your
unicorn matches the phenotype!!!
5. Choose a piece of string so you can hang your suncatcher up!
Trait Genotype Phenotype
Standing on 4 or 2