Punnett Square Notes

What is Genetics? Genetics is the scientific study of heredity

What is a Trait? A trait is a specific characteristic that varies from one individual to another. Examples: Brown hair, blue eyes, tall, curly

What is an Allele? Alleles are the different possibilities for a given trait. Every trait has at least two alleles (one from the mother and one from the father) Example: Eye color – Brown, blue, green, hazel Examples of Alleles: A = Brown Eyes a = Blue Eyes B = Green Eyes b = Hazel Eyes

What are Genes? Genes are the sequence of DNA that codes for a protein and thus determines a trait.

Gregor Mendel Father of Genetics 1st important studies of heredity Identified specific traits in the garden pea and studied them from one generation to another

Mendel’s Conclusions Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring Law of Independent Assortment – Genes for different traits are inherited independently of each other

R r Dominant vs. Recessive Dominant - Masks the other trait; the trait that shows if present Represented by a capital letter Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present Represented by a lower case letter R r

Dominant & Recessive Practice TT - Represent offspring with straight hair Tt - Represent offspring with straight hair tt - Represents offspring with curly hair T – straight hair t - curly hair

Genotype vs. Phenotype Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has. Example: Tt, ss, GG, Ww Phenotype – The physical characteristics of an organism; The way an organism looks Example: Curly hair, straight hair, blue eyes, tall, green

Homozygous vs. Heterozygous Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt) Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR rr Rr

Punnett Squares Punnett Square – Diagram showing the gene combinations that might result from a genetic cross Used to calculate the probability of inheriting a particular trait Probability – The chance that a given event will occur

Punnett Square Parent Offspring Parent

How to Complete a Punnett Square

Y-Yellow y-white Genotype: 1:2:1 (YY:Yy:yy) Phenotype: 3 Yellow 1 White

You Try It Now! Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)

TT x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T T t

TT x tt Step Two: Complete the Punnett Square T T t Tt

TT x tt Tt t Genotype: 4 - Tt Phenotype: 100% Tall Step Three: Write the genotype and phenotype T T t Genotype: 4 - Tt Phenotype: 100% Tall Tt Remember: Each box is 25%

You Try It Now! Give the genotype and phenotype for the following cross: Tt x tt

Tt x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T t t

Tt x tt Step Two: Complete the Punnett Square T t t Tt tt

Tt x tt Tt tt t Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: Step Two: Complete the Punnett Square T t t Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short Tt tt Remember: Each box is 25%