Punnett Squares

Course: Ag Biology

Unit: Genetics

Department: Agriculture

Teacher: Mrs. Raggio

School: Chowchilla Union High School

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.

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

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

t

​

TT x tt

Step Three: Write the genotype and phenotype

​

​

T T

t

t

​

Genotype:

4 - Tt

​

Phenotype:

100% Tall

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

t

​

Tt x tt

Step Two: Complete the Punnett Square

​

​

T t

t

t

​

Tt x tt

Step Two: Complete the Punnett Square

​

​

T t

t

t

​

Genotype:

Tt - 2 (50%)

tt - 2 (50%)

​

Phenotype:

50% Tall

50% Short

Remember: Each box is 25%

Some Terminology

• P₁ – Original parents
• F₁ – First generation
• F₂ – Second generation

​

• P₁ X P₁ = F₁
• F₁ X F₁ = F₂

Incomplete Dominance

• 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

• 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

• 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

• 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 (I^A, I^B, and i), �which results in four �different possible blood �types
• Hair Color – Too many �alleles exist to count
• There are over 20 �different shades of �hair color.

​

Multiple Alleles

• There Are 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

• 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.

​