Probability
and
Punnett Squares
Please take out
your snowbaby,
science journal
and any missing
work (graphs)?

Bunny Punnett Square
5:03 minutes
Eye Color Punnett Square
4:00 minutes
PUNNETT SQUARES
PROBABILITY
 Probability
is the likelihood that an event will
occur. What chance does it have of happening?
 Punnett
squares are a useful tool for
predicting what the offspring will look like
when mating plants or animals.
 Punnett
squares only show the probability
(chance) of the offspring, NOT exactly what
will happen.
 Reginald
Crundall Punnett, a mathematician,
came up with these in 1905, long after
Mendel's experiments.
 Phenotype
- An organism’s physical
appearance or visible traits. What can be
seen. (ex: red flowers, or white flowers)
 Genotype
- An organisms genetic makeup or
allele combination. What can not be seen.
(ex:Aa, AA, aa)
 Allele
– 2 forms of a gene, 1 from each parent.
 Allele
- Alphabet
PHENOTYPE AND GENOTYPE
 Heterozygous
- Also known as a hybrid, is
when the allele combination is one of each,
one dominant and one recessive. (Aa)
 Homozygous-
is when the allele combination is
the same, both dominant (AA) or both
recessive (aa).
HETEROZYGOUS AND HOMOZYGOUS
G = grey fur
g = white fur
3 crosses have a dominant allele (GG, Gg,
and Gg) and 1 cross has 2 recessive alleles
(gg) so 75% have the chance to have gray
fur and 25% have the chance to have
white fur.
 Punnett
 You
Square Practice Page
can try these at home if you need more practice!
 Let’s
do some together.
 Let's
say that in seals, the gene for the length of the
whiskers has two alleles. The dominant allele (W) codes
long whiskers & the recessive allele (w) codes for short
whiskers.
 a)
What percentage of offspring would be expected to
have short whiskers from the cross of two long-whiskered
seals, one that is homozygous dominant and one that is
heterozygous?
 b)
If one parent seal is pure long-whiskered and the other
is short-whiskered, what percent of offspring would have
short whiskers?
 A)
What percentage of offspring would be expected to
have short whiskers from the cross of two long-whiskered
seals, one that is homozygous dominant and one that is
heterozygous?
W
W
a) W = allele for long whiskers
w = allele for short whiskers
homozygous dominant seal = WW
heterozygous seal =Ww
ANSWER: 0%
w
WW
Ww
P-SQUARE PRACTICE QUESTION #1A
w
WW
Ww



b) If one parent seal is pure long-whiskered and the other is short-whiskered, what
percent of offspring would have short whiskers?
ANSWER: 0%.
Again, I suggest starting by defining symbols like so:
W = allele for long whiskers
w = allele for short whiskers
"Pure" is the same as homozygous, so "pure long-whiskered" would be "WW".
If you're a seal, the only way to have short whiskers is to have the homozygous
recessive genotype, in other words be "ww".
So our cross is: WW x ww.
w
w
W
W
Ww
Ww
Ww
Ww
P-SQUARE PRACTICE QUESTION #1B
P-SQUARE PRACTICE QUESTION #2
 In
purple people eaters, one-horn is dominant and
no horns is recessive. Draw a Punnet Square
showing the cross of a purple people eater that is
hybrid for horns with a purple people eater that
does not have horns. Use H and h for your alleles.
Summarize the genotypes & phenotypes of the
possible offspring.
P-SQUARE PRACTICE QUESTION #2

In purple people eaters, one-horn is dominant and no horns is
recessive. Draw a Punnet Square showing the cross of a purple
people eater that is hybrid for horns with a purple people eater
that does not have horns. Use H and h for you alleles. Summarize
the genotypes & phenotypes of the possible offspring.
What are the alleles for the hybrid
parent?
Hh
What are the alleles for the parent
without horns?
Both recessive, so hh
What are the genotypes?
50% hybrid (Hh), 50% homozygous
recessive (hh)
What are the phenotypes?
50% have 1 horn and 50% do not.
h
h
H
h
Hh
hh
Hh
hh

Now it is your turn with some sample problems. Have fun! Use the back
of this sheet to answer the questions below. Don’t forget to determine
your alleles first and then make your Punnett Square.
One unicorn carries
heterozygous, straight horned
traits, and its mate carries
homozygous spiral horned traits.
Use a Punnett Square to
determine the probability (what
percentage) of one of their
offspring having a spiral horn. Use
any letter you’d like to represent
the alleles.
One
Sasquatch carries
heterozygous, long-haired traits,
and its mate carries homozygous
short-haired traits. Use a Punnett
Square to determine the
genotypes & phenotypes of the
possible offspring. Use any letter
you’d like for the alleles. Then
summarize the phenotypes and
genotypes.