Punnett Square Notes
Crossing Traits
• We use Punnett Squares to cross parents traits.
This gives us a percentage to be able to
determine an offspring’s possible traits.
• We represent traits with letters.
• Dominant traits are represented with a capital
letter.
• Recessive traits are represented with a lower
case letter.
• Since each parent passes on half of an
offsprings traits, each person has two letters,
one from mom and one from dad. We refer
to these letters as “alleles”.
Expressing Traits
• If an offspring possesses the dominant
trait, it can be expressed one of two
ways:
DD
or
Dd
• Recessive traits can only be expressed
as:
dd
Naming
• Each one of these ways has a specific name.
– DD  homozygous dominant
both alleles are the same
a dominant gene was passed on
– Dd  heterozygous OR heterozygous dominant
alleles are different
a dominant gene was passed on
– dd  homozygous recessive
both alleles are the same
only recessive genes were passed on
Other Vocab
• purebreed: homozygous dominant or
homozygous recessive
• hybrid: heterozygous
• genotype: trait represented by letters
– Genetic makeup
• phenotype: the physical
representation of the genotype
– How we see the traits
Putting it all together
Example 1:
Trait: Dimples
(having dimples is dominant, NOT having dimples is recessive)
Mom: homozygous dominant
-genotype: DD
-phenotype: dimples
Dad: homozygous recessive
-genotype: dd
-phenotype: NO dimples
Creating the Punnett Square
D
d
d
D
Parent 1 Genotypes
Dd Dd
Dd Dd
Parent 2 Genotypes
Possible genotypes for offspring: Dd
Possible phenotypes: dimples
Capital letters
are ALWAYS
written first!!
Putting it all together
Example 2:
Trait: Freckles
(having freckles is dominant, NOT having freckles is
recessive)
Mom: homozygous recessive
-genotype: ff
-phenotype: NO freckles
Dad: heterozygous
-genotype: Ff
-phenotype: freckles
Creating the Punnett Square
F
f
f
f
Possible genotypes for offspring:
Ff
ff
Ff
Ff
Possible phenotypes:
freckles
no freckles
ff
ff
What are the chances of a child
having freckles?
2 out of 4 OR 50%
What are the chances of a child
NOT having freckles?
2 out of 4 OR 50%