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 Remember: Each box is 25% Genotype: 4 - Tt Phenotype: 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 Remember: Each box is 25% Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short

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

Incomplete Dominance - Situation in which one allele is not completely dominant over another. Example – Red and white flowers are crossed and pink flowers are produced.

Codominance - Situation in which both alleles of a gene contribute to the phenotype of the organism. Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan). +

Multiple Alleles- Three or more alleles of the same gene. Even though three or more alleles exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.

Examples of Multiple Alleles 1. Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.

Examples of Multiple Alleles 2. Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types 3. Hair Color – Too many alleles exist to count There are over 20 different shades of hair color.

Multiple Alleles There Always Multiple Alleles! Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see. But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene. We probably know this already - as we look around at other people, we see infinite variation.

Polygenic Trait - Trait controlled by two or more genes. Polygenic traits often show a wide range of phenotypes. Example: The wide range of skin color in humans comes about partly because more than four different genes probably control this trait.