1. Probability and Punnett Squares

2. Probability
Probability-the chance a particular outcome will occur, usually expressed in a percent, like there is a 40% chance of snow today.
Example- when you flip a coin, there is a 50% chance of getting heads and a 50% chance of getting tails

3. Mendel and Probability
Mendel was the first to realize that he could use probability to predict the chances a particular genetic outcome could occur, like the chances of two plants producing tall or short offspring.

4. When Mendel crossed his purebred tall plants and his purebred short plants, his F1 generation were 100% tall plants.
He was surprised that when he crossed the F1 generation, he got 75% tall plants and 25% short plants.

5. Punnett Squares
A tool to use to predict the probabilities of possible genotypes of offspring
Parent genotypes are put above the square and on the side
Fill in letters across and down to show the possible probabilities of offspring allele combinations.

6. Dad Pea Plant: Homozygous dominant (TT)
Mom Pea Plant: Homozygous recessive (tt)
First, fill in Dad’s alleles across top of the square T T
Then, fill in Mom’s alleles down the side of the square
Dad 
Mom  t t

7. First, fill the dad’s alleles down the columns.
Then, fill the mom’s alleles across the rows. t T t T t t T t T t
*Remember-an organism gets two copies of every gene, one from their dad and one from their mom. Also, Tt is the same as tT, but try to get in the habit of writing the capital letter first.

8. T T
Genotype: 100% Tt
Phenotype: 100% tall t T t T t t T t T t
Results: All 4 of the squares have the heterozygous combination. So, there is a 100% chance of having a plant with a heterozygous genotype. There is also a 100% chance to have an offspring with the tall phenotype.

9. Now try to fill in the Punnett Square for Mendel’s F1 Generation on your handout: Remember, the genotypes for all the plants was Tt. So fill in the Punnett Square and predict the possible outcomes of this cross. Give % for both genotype and phenotype.

10. T t
Click to check your answers-Were you right? T T T T t T t t t t
Genotypes: 25% TT, 50% Tt, and 25% tt
Phenotypes: 75% tall, 25% short

11. Now you are Punnett Square Experts
Now you are Punnett Square Experts! Good luck predicting the outcomes of genetic crosses!!!