Punnett Square Notes What is Genetics Genetics is

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

Mendel’s Conclusions 1. Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring 2. Law of Independent Assortment – Genes for different traits are inherited independently of each other

Dominant vs. Recessive Dominant - Masks the other trait; the trait that shows if present Represented by a capital letter R 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

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

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 T

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

TT x tt Step Three: Write the genotype and phenotype t T T Tt Tt Tt Genotype: 4 – Tt 0 -TT 0 -tt Phenotype: Remember: Each box is 25% 100% Tall

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 t

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

Tt x tt Step Two: Complete the Punnett Square t Tt tt Genotype: Tt - 2 (50%) tt - 2 (50%) TT-0 Phenotype: 50% Tall Remember: Each box is 25% 50% Short

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond. R r r r Rr rr What percentage of the offspring will have red hair? 50%

Let’s try some more… In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a short pea plant. t t Tt tt What are the percentage of phenotypes? 50% tall 50% short

Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes. r r R Rr Rr What is the possibility of a red eye off springs? 0%

Some Terminology P 1 – Original parents F 1 – First generation F 2 – Second generation P 1 X P 1 = F 1 X F 1 = F 2