Introduction to Punnett Squares
I. Vocabulary
 Dominant- an allele for a gene that masks
other alleles; Symbolized by a capital letter
 Recessive- an allele that is masked by a
dominant allele; Symbolized by a lower case
letter.
Vocab Cont.
 Homozygous/Purebred- An organism with
two of the same alleles for a gene
 Heterozygous/Hybrid- An organism with two
different alleles for a gene
Vocab Cont.
 Phenotype- What a trait appears like for an
organism (Picture)
 Genotype- the alleles an organism has for a
gene written out in symbols
II. Examples of Phenotype & Genotype
Phenotype
Homozygous Tall
Tall
Genotype
TT
Heterozygous
Tall
Tt
Homozygous Short
Short
tt
 How can we predict the result of a cross
without actually performing the cross?
 Scientists use punnett squares
III. Drawing a Punnett Square

For this example, we will use Mendel’s
second experiment:
F1 offspring X
1.
F1 offspring = ?
Determine what symbols to use:
Dominant = Tall =T
Recessive = Short =t
2. Write out the genetic cross using symbols
F1 offspring x F1 offspring = ?
Tt
x
Tt
=?
Tt x Tt = ?
3. Draw the Punnett Square
Tt x Tt = ?
4. Add the genotypes from the genetic cross
above. Use 1 letter per square
T
t
One goes across
the top
T
t
One goes down the side
Tt x Tt = ?
5. Put the symbol on top into the squares below it
T
t
T
t
T
t
T
t
Tt x Tt = ?
6. Put the symbol on the left into the squares next
to it
T
t
T TT
Tt
t tT
tt
Tt x Tt = ?
7. List the possible genotypes and phenotypes of
the offspring
T
t
Genotype
Phenotype
TT
Tall
T TT Tt
Tt
Tall
tt
Short
t tT tt
Tt x Tt = ?
8. Calculate the probability of phenotypes and
genotypes
T
t
Each square represents a
T TT Tt
¼ or 25% probability
t tT
tt
Tt x Tt = ?
T
t
T TT
Tt
t tT
tt
Genotype
TT
Tt (tT)
tt
Probability
¼
2/4 = ½
¼
Tt x Tt = ?
T
t
T TT
Tt
t tT
tt
Phenotype
Tall (TT, Tt)
Short (tt)
Probability
¾
¼