1. Punnett Square Notes

2. 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”.

3. Expressing Traits If an offspring possesses the dominant trait, it can be expressed one of two ways: DDorDd Recessive traits can only be expressed as: dd

4. 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

5. 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

6. 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

7. Creating the Punnett Square Dd Dd Dd Dd Dd Dd Dd Dd Parent 1 Genotypes Parent 2 Genotypes D dddd Capital letters are ALWAYS written first!! Possible genotypes for offspring: Dd Possible phenotypes: dimples

8. 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

9. Creating the Punnett Square Ff ff f FfFf 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%