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

– Dd heterozygous OR heterozygous dominant

– dd homozygous recessive

both alleles are the same a dominant gene was passed on

alleles are different a dominant gene was passed on

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

Dd Dd

Dd Dd

Parent 1 Genotypes

Parent 2 Genotypes

D D

d

d

Capital letters are ALWAYS written first!!

Possible genotypes for offspring: DdPossible phenotypes: dimples

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

Ff Ff

ff ff

f f

F

f

Possible genotypes for offspring: Ff ff

Possible phenotypes: freckles no freckles

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%