1. Probability & Punnett Squares

2. Probability
A concept that can be used to predict the results of a particular event
Examples
Chance of a specific team winning a sporting event
Chance of a coin landing on heads in a coin toss
Predict what is likely to occur, not necessarily what will actually occur

3. Probability and Heredity
Think back to Mendel’s plant experiments
He noticed that traits were inherited in patterns
For example, when he crossed two plants that were heterozygous for stem height (Tt) he noticed their offspring would inherit this trait in a predictable pattern, with 3 out of 4 having tall stem height.

4. Probability and Heredity
Each time Mendel repeated the cross, he would obtain similar results
He could say that the probability, or chance, of the cross producing a tall plant was 3 in 4 and the probability of producing a short plant was 1 in 4.

5. Mendel noticed the same pattern of inheritance in other traits as well

6. Probability and Punnett Squares
Punnett Square – A chart that shows how parents’ alleles might combine in an offspring
a tool that can help you understand the patterns of heredity
Geneticists use Punnett Squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome
Punnett Squares Explained at

7. Probability and Punnett Squares
What is the probabilty of…..
The offspring having a yellow seed color?
2 in 4
The offspring having a green seed color?

8. Probability and Punnett Squares
How else can probability be written?
The offspring having a yellow seed color?
2 in 4 or 50%
The offspring having a green seed color?

9. Using a Punnett Square
In rabbits, black fur color is dominant to white. What is the probability of producing a white rabbit if two heterozygous rabbits mate?

10. Using a Punnett Square Step 1: figure out the genotype of the parents.
Black is dominant to white.
B - black
b - white
Parents are heterozygous.
Bb - dad
Bb - mom

11. Using a Punnett Square Bb Bb
Step 2: set up a Punnett Square.
B b B b

12. Using a Punnett Square
Step 3: Fill in the Punnett Square.
B b B b

13. Using a Punnett Square
Step 3: Fill in the Punnett Square.
B b BB B b

14. Using a Punnett Square B b B b BB Bb
Step 3: Fill in the Punnett Square.
B b BB Bb B b

15. Using a Punnett Square B b B b BB Bb Bb
Step 3: Fill in the Punnett Square.
B b BB Bb B Bb b

16. Using a Punnett Square B b B b BB Bb bb Bb
Step 3: Fill in the Punnett Square.
B b BB Bb B bb Bb b

17. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
B b BB Bb B bb Bb b

18. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
2 heterozygotes
B b BB Bb B bb Bb b

19. Using Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
B b BB Bb B bb Bb b

20. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Phenotypes
B b BB Bb B bb Bb b

21. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Phenotypes
3 black fur
B b BB Bb B bb Bb b

22. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Phenotypes
3 black fur
1 white fur
B b BB Bb B bb Bb b

23. Probability and Punnett Squares
In rabbits, black fur color is dominant to white. What is the probability of producing a white rabbit if two heterozygous rabbits mate?
1 in 4 or 25%

24. Representing Ratios Probability can also be represented in ratios
A ratio compares or shows the relationship between a part to the whole
Example the probability that a coin will land on heads in a single coin toss is 1 in 2 or 50%
As a ratio, the probability would be written as 1:2 and would be read as, “One to two.”
In genetics we use rations to represent probability

25. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes =
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Genotypic Ratio = 1:2:1
Phenotypes
3 black fur
1 white fur
B b BB Bb B bb Bb b

26. Using a Punnett Square B b B b BB Bb bb Bb Step 4: Count the results.
Genotypes =
1 homozygous dominant
2 heterozygotes
1 homozygous recessive
Genotypic Ratio = 1:2:1
Phenotypes
3 black fur
1 white fur
Phenotypic Ratio = 3:1
B b BB Bb B bb Bb b

27. Let’s Practice
In pine trees, long needles are dominant over short needles. Cross a homozygous dominant plant with a plant that is heterozygous. What is the probability that the parents will produce offspring that have short needles? Give the genotypic and phenotypic ratios.

28. Let’s Practice
2. In cats, a striped coat is dominant over a solid coat. Cross a cat with a solid coat with a cat that is heterozygous for a striped coat. What is the probability that the parents will produce offspring with a solid coat? Give the genotypic and phenotypic ratios.

29. Let’s Practice
3. In humans, dark hair is dominant over light hair. Cross a man that is homozygous recessive with a woman that is homozygous dominant. What is the probability that the parents will produce offspring with light hair? Give the genotypic and phenotypic ratios.