Medelian Genetics
and
How to use a Punnett Square
to predict probable offspring
Simple Steps to follow
1. Determine the parental phenotype and
genotypes of the traits being crossed.
2. Determine the possible gametes from the
parents in #1.
3. Use the gametes to fill the outside of a
blank Punnett Square.
4. Complete the inside of the Punnett
Square.
More Steps…
5. Count the number of possible genotypes
found in the Punnett Square.
6. Determine the percentage of genotypes
found.
7. Determine the phenotypes (based on the
genotypes) and percentages from the
Punnett Square.
8. Celebrate – you’re done!
Example. . .
Oompah Loompah Genetics
Who/What are Oompah
Loompahs?
Oompah Loompah
Dad
Step 1.
Mom
Phenotype = Face Color (F)
Dominant =
BLUE
Recessive =
ORANGE
Genotype = Genetic make-up
Dominant =
FF or Ff
Recessive =
ff
Step 2 . Determine possible gametes
Genotype = FF or Ff
Gamete = F
F or f
Genotype = ff
Gamete = f
Let’s use the heterozygous dad to find out what possible offspring
they might have.
Step 3. Fill in gametes outside of Punnett
Square
Mom
f
F
f
Dad
f
Step 4. Fill in inside of Punnett Square
F
f
f
f
Ff
Ff
ff
ff
Step 5. Count the number of possible offspring
genotypes found in the Punnett Square.
Genotypes:
f
f
F
Ff
Ff
f
ff
ff
Ff =
2 out
of 4
= 50%
ff =
2 out
of 4
= 50%
Step 6: Determine
percentages - done above!
Step 7. Determine offspring phenotypes and
percentages
Genotypes are:
Ff = 2/4 = 50% and ff = 2/4 = 50%
Phenotypes are:
Ff = BLUE face = 2/4 = 50%
ff = ORANGE face = 2/4 = 50%
Summary
Crossing a blue face
Oompah Loompah
with an orange face
Ompah Loompah
may produce 50%
blue-faced
children and 50%
orange-faced
children
Make Sense???
Try the Oompah Loompah worksheet!