Probability & Punnett Squares

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 http://www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html

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? 2 in 4

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? 2 in 4 or 50%

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 • Step 3: Fill in the Punnett Square. B b BB Bb B b

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

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

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

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

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

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

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

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

26. 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 Using a Punnett Square 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.