How To Use a Punnett Square

1
How To Use a Punnett Square

• Predicting Genetic Outcomes

2
What Is a Punnett Square?

• Punnett squares are used to predict the possible
  outcomes of genetic crosses
• Only predicts PROBABILITIES, no guarantees!

3
Setting Up the Punnett Square

• Determine what possible alleles the parents have.
• Place one parents alleles on the top of the
  Punnett Square.
• Place the other parents alleles on the left side
  of the Punnett Square.

4
Example

• Mom is heterozygous for brown hair, meaning she
  has one dominant allele (B) and one recessive
  allele (b).
• Dad is homozygous recessive, meaning he has two
  recessive alleles (bb)

5
Example
B
b
Bb bb
Bb bb
b
b
6
Results
B
b
Bb bb
Bb bb
b
b
Genotypes 50 Bb 50 bb
Phenotypes 50 brown hair 50 blonde hair
7
Practice

• 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).

8
Practice answers
B
b
Genotypes 25 BB 50 Bb 25 bb
BB Bb
Bb bb
B
Phenotypes 75 brown eyes 25 blue eyes
b