I. Punnett Squares
Punnett Squares &
Probability
How to figure out what the
probable outcome of a genetic
cross will be
A. Vocabulary
1. Genotype=
Genotype=
genetic makeup, what the genes say
2. Phenotype=
Phenotype=
physical appearance, how it looks
3. Genotypic ratio=
ratio=
(3 #s) homozygous dominant:
dominant: heterozygous:
heterozygous: homozygous recessive
4. Phenotypic ratio=
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
D. Example 1
ƒ
Let T = tall pea plant
ƒ
Let t = short pea plant
ƒ
Cross a short pea plant
with a heterozygous pea plant
C. Punnett Square Conventions
1. A capital letter stands for the dominant
trait (lower case = recessive)
recessive)
2. If there is a capital letter, write it first
3. First parent mentioned is the ♂.
Write that genotype at top of Punnett.
Punnett.
D. Example 1
3. Draw and label square
1. What is the cross?
2. tt x Tt
http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm
http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm
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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
2
E. Example 2
3. Fill in squares
T
4. Genotypic &
Phenotypic ratios
t
gr =
T
TT
Tt
t
Tt
tt
1:2:1
pr =
3:1
Statements about Probability
2. Previous events do not
affect future outcomes.
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
Bibliography
Punnett baby steps http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm 11/4/07
3. The larger the number of trials, the closer
one gets to the expected probability.
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