Punnett Squares &
Probability
How to figure out what the
probable outcome of a genetic
cross will be
I. Punnett Squares
A. Vocabulary
1. Genotype=
genetic makeup, what the genes say
2. Phenotype=
physical appearance, how it looks
3. Genotypic ratio=
(3 #s) homozygous dominant: heterozygous: homozygous recessive
4. Phenotypic ratio=
(2 #s) dominant appearance : recessive appearance
B. Punnett Square Procedure
1.
2.
3.
4.
5.
Figure out the cross
Write down what the cross is
Draw & label the Punnett Square
Fill in the squares
Write down the genotypic & phenotypic
ratios
C. Punnett Square Conventions
1. A capital letter stands for the dominant
trait (lower case = recessive)
2. If there is a capital letter, write it first
3. First parent mentioned is the ♂.
Write that genotype at top of Punnett.
D. Example 1

Let T = tall pea plant

Let t = short pea plant

Cross a heterozygous pea plant
with a short pea plant
1. What is the cross?
2. Tt x tt
D. Example 1
3. Draw and label square
http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm
D. Example 1
4. Fill in the squares
D. Example 1
4. Fill in the squares
D. Example 1
4. Fill in the squares
D. Example 1
5. Figure out the genotypic ratio
Homozygous dominant = none
Heterozygous = 2
Homozygous recessive = 2
genotypic ratio = none : 2 : 2
D. Example 1
Figure out the phenotypic ratio
Dominant appearance: recessive appearance
2:2
reduce to 1 : 1
E. Example 2
Cross two pea plants heterozygous for tallness
1. Tt x Tt
2. Draw and label square
T
T
t
t
E. Example 2
3. Fill in squares
T
4. Genotypic &
Phenotypic ratios
t
gr =
T
TT
Tt
1:2:1
pr =
3:1
t
Tt
tt
II. Probability
A. Probabilty =
the likelihood that a particular event
will occur
B. Statements about probability
1. probability =
# of times an event occurs
# of trials
Statements about Probability
2. Previous events do not
affect future outcomes.
3. The larger the number of trials, the closer
one gets to the expected probability.
Bibliography
Punnett baby steps http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm 11/4/07