Download How To Use a Punnett Square

How To Use a
Punnett Square
Predicting Genetic Outcomes
QQ#1
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Write down anything that comes to mind
about Punnett Squares.
On a scale from 1-5, how well could you
explain/complete a Punnett Square-why (I
wont ask you to share this with the class)
What Is a Punnett Square?
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
Punnett squares are used to predict the
possible outcomes of genetic crosses
Only predicts PROBABILITIES, no
guarantees!
QQ#2
Why might we want to predict the probability
of genetic crosses?
Setting Up the Punnett Square
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1. Determine what possible alleles the parents have.
2. Draw a square, and within the square divide it into 4
equal squares
3. Place the male’s alleles on the top of the Punnett
Square.
4. Place the female’s alleles on the left side of the
Punnett Square.
5. Like a multiplication table, fill in the offspring possible
genotypes
Example
Example

Female is heterozygous for brown hair,
meaning…
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she has one dominant allele (B) and one
recessive allele (b).
Male is homozygous recessive, meaning…

he has two recessive alleles (bb)
Example
b
b
B
Bb
Bb
b
bb
bb
Results
B
b
Bb
bb
Bb
bb
b
Genotypes –
50% Bb 50% bb
Phenotypes –
50% brown hair
b
50% blonde hair
Practice
QQ#3
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Use a Punnett Square to determine the
possible genotypes and phenotypes of the
offspring from the following cross.
Brown eyes (B) are dominant over blue
eyes (b). Cross two heterozygous parents
(Bb).
Practice answers
B
b
Genotypes
25% BB
50% Bb
B
BB
Bb
25% bb
Phenotypes
b
Bb
bb
75% brown eyes
25% blue eyes