Punnett Square Notes Gregor Mendel Father of Genetics

Punnett Square Notes

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

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: B = Brown Eyes b = Blue Eyes G = Green Eyes g = Hazel Eyes

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

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

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). +

Dominant & Recessive Practice Upper case letters represent Dominant traits. Lower case letters represent Recessive traits. TT - Represent offspring with straight hair Tt - Represent offspring with straight hair tt - Represents offspring with curly hair T – straight hair t - curly hair

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

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

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

Let’s put a couple examples in our notebook!

You Try It Now! Complete the following punnett square problems in your SISN. Create a punnett square and 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 Tt Tt

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

You Try It. Complete the following punnett square problems in your SISN. 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 t Tt tt

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

Work through the practice!!!

Punnett Square Pop Quiz Take out a blank sheet of notebook paper and something to write with.

1. What is the dominant gene? What is the recessive gene? B – Brown eyes b – Blue Eyes

2. Label which is heterozygous and which is homozygous. a. BB b. Bb c. bb

3. A scientist crossed a homozygous tall daisy with a homozygous short daisy. Each of the daisies has two of the same alleles that make them homozygous. The tall trait of the daisy is the dominant, and is represented with a “D”. The short trait of the daisy is the recessive trait, and is represented with a “d”. Use a Punnett Square to cross a homozygous tall daisy (DD) with a homozygous short daisy (dd).

4. In the first question, what is the genotype and phenotype of all the 1 st generation offspring? Genotype ________ Phenotype ________

5. Black fur (B) in guinea pigs is dominant over white fur (b). Create a punnett square that displays the offspring if a heterozygous black fur guinea pig is crossed with a homozygous white fur guinea pig.

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

Blood type punnett squares

Blood Type Punnett Square A man who is AB marries a woman who is heterozygous type B. If they had four children, what would their probable genotypes and phenotypes be? *hint – you will need to create a punnett square.

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.

Blood type