1. Punnett Square Notes
By Ms. Duko

2. What is Genetics?
Genetics is the scientific study of heredity

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

4. 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

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

6. 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

7. 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

8. 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)

9. 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

10. Punnett Square
Parent
Offspring
Parent

11. How to Complete a Punnett Square

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

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

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

15. 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%

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

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

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