1. Probability and Punnett Squares
Introduction to Genetics
Honors Biology
Chapter 11 Section 2

2. Genetics and Probability
The likelihood that a particular event will occur is called probability
Ex: Flip a coin, the probability it will land on heads is 1 in 2. This is ½ or 50%
Flip the coin 3 times in a row and the probability of flipping 3 heads in a row is 1/8
½ x ½ x ½ = 1/8
You have 1 chance in 8 of flipping heads 3 times in a row
A coin is similar to genetics because alleles separate randomly, just like flipping a coin. We can predict the outcomes of genetic crosses (Punnett Squares)

3. Punnett Squares
The types of gametes that can be produced from one parent are on one side and the gametes from the other parent are lined up along the other side
The possible combinations of the F2 generation are found inside the squares
The letters represent alleles. Capitol letters are dominant and little letters are recessive
EX: plant height tall = TT or Tt Short = tt

4. Cross between hybrid tall pea plants
Mom Tt
Dad Tt

5. Cross between hybrid tall pea plants
Mom Tt
Dad Tt

6. Punnett Squares
The square can be used to predict and compare genetic variations that result from a cross
Organisms with 2 identical alleles for a trait (TT or tt) are called homozygous
Organisms with 2 different alleles for a trait (Tt) are called heterozygous

7. Punnett Squares
All tall plants have the same phenotype, or physical characteristics
They do not contain the same genotype, or genetic makeup
EX: a tall plant
phenotype is tall
genotype is TT or Tt

8. Probability and Segregation
In the F1 heterozygous hybrid cross, ¾ of the plants should be tall
¼ TT
½ (¼ + ¼) Tt
¼ of the plants should be short
¼ tt
The 3:1 ratio of dominant to recessive
Segregation occurred according to Mendel’s model

9. Probabilities Predict Averages
The more offspring, the closer you will come to reaching your predicted outcomes. Hundreds and Thousands of individuals
The same can be applied to coin flipping
This is because probabilities predict the outcome of a LARGE number of events

10. Frayer Model

11. Frayer Model Assignment
Create a Frayer Model for each of the following words
Trait
Phenotype
Genotype