Punnett Squares
Cornell Notes Page 47
For what is a • to show all possible
Punnett
ways alleles from
Square Used?
parents can combine
to create offspring
• to predict the
probability of
offspring inheriting a
particular trait.
• Parents’ genotypes go
How is a
along outside of box
Punnett
Square Used? • Offspring genotypes are
written inside each box
• Each box represents
25% of possible
offspring
• Look at each genotype
to determine phenotype.
Practice Using a PUNNETT
SQUARE
One of Harry Potter’s best friends is
Hermione Granger. Hermione is a wizard,
but her parents are not. So, how did
Hermione get the wizarding trait? We
can use a Punnett square to figure it out…
Using a PUNNETT SQUARE
A letter is chosen to represent a
characteristic, such as magical powers.
Having magical powers is a recessive trait,
so we’ll use:
• M for muggle (non-wizard)
• m for wizard
Using a PUNNETT SQUARE
Both of Hermione’s parents are muggles
and both have a genotype of Mm.
Genotypes: Mm x Mm (mom x dad)
Phenotypes: both muggles (non-wizards)
Using a PUNNETT SQUARE
Cross Hermione’s parents:
M
M
m
m
Using a PUNNETT SQUARE
The cross looks like this:
M m
M MM Mm
m Mm mm
Using a PUNNETT SQUARE
Each square stands for 25% of the
possible offspring. Or, ¼ of the possible
offspring.
M m
M MM Mm
m Mm mm
Using a PUNNETT SQUARE
Remember, the recessive trait is having
magical powers. What’s the probability of
being a wizard?
M m
M MM Mm
m Mm mm
Expressing PROBABILITIES
Can express probability several ways:
3:1 ratio of muggles to wizards
1/4 chance of being wizard
25% chance of being wizard
M m
M MM Mm
m Mm mm
Expressing PROBABILITIES
can use for genotypes
25% - MM
50% - Mm
25% - mm
or phenotypes
M
3/4 - muggles
1/4 – wizards
m
3:1 muggles
to wizards
M m
MM Mm
Mm mm
Practice #1
Mendel crossed a pure-breeding purple flower
(PP) with a pure-breeding white flower (pp).
Complete a Punnett square for this cross.
A. Which is the dominant trait?
B. What are the offspring genotypes?
C. For the offspring, what fraction will have
purple flowers?
D. What percentage will have genotype Pp?
Widow’s Peak
Widow’s Peak
No Widow’s Peak
Practice #2
A mom with a widow’s peak (Ww) is crossed
(mates) with a dad who does not have a
widow’s peak (ww). Complete a Punnett
square for this cross.
A. Which is the dominant trait?
B. What are the offspring genotypes?
C. What is the ratio of widow’s peak to no
widow’s peak for the offspring?
D.What is the ratio of Ww to ww for the
offspring?
Practice #3
A man and a woman both have a cleft chin.
Both have genotypes of Cc. Complete a
Punnett square for a cross between them.
A. Which is the dominant trait?
B. What are the offspring genotypes?
C. What fraction of offspring will have a cleft
chin?
D. What percentage will have genotype Cc?