Section 11-2: Probability and Punnett Squares 1. Probability • The likelihood that a particular event will happen – For example: A Coin Toss – 2. A coin has a 50% chance to be heads or tails • 3. Is the outcome of the flip determinant on past flips? – No! Probability and Genetics • How does probability effect genetics? • The Segregation of Alleles – 4. When alleles segregate it is completely random, very similar to a coin toss. – Interesting to think of how things could have been different don’t you think? The Punnett Square • 5. Allows possible gene combinations to be predicted by the use of a diagram • Can be used to predict and compare genetic variations that will result from a cross – As well as the probability of occurance Punnett Square Diagram Dominant Alleles T T t Recessive Alleles t 8. Homozygous • Having two identical alleles for a particular trait – Ex: TT or tt • “True-Breeding” for • a Particular Trait – Examples: • Hydras • Mushrooms • Bacteria 10. Heterozygous • Organisms having two different alleles for the same trait – Ex: Tt • “Hybrids” for a particular trait – Examples: • • • • Cross Pollinating Plants Mules The majority of higher organisms 9. Phenotype • The physical characteristics of an organism • In our Punnett Square example what are our phenotypes? – Tall • TT or Tt – Short • tt 7. Genotype • The genetic makeup of an organism • In our Punnett Square example what would the genotypes be? – TT- 25% – Tt- 50% – tt- 25% Phenotype VS Genotype: Plant 1: Homozygous (TT) Plant 2: Heterozygous (Tt) 12. Just because an organism has the same phenotype, that doesn’t mean it always has the same genotype. 13.Probability and Segregation – Let’s look at our Hybrid Cross results: – 14. Mendel’s Requirements for Segregation • ¾ of plants displayed the Tall phenotype • ¼ of plants displayed the Short phenotype • 3:1 ratio of tall to short plants – Did Segregation occur in relation to Mendel’s model? • YES! Probabilities Predicting Averages • 15.Probabilities predict the average outcome of large scale events, BUT not precise individual events • 16. For example: Coin Toss – You can get 50:50 ratio but it could take you numerous tosses • Are the odds in our favor for just 2 tosses? Probabilities Predicting Averages • 17. The larger the number of offspring in a genetic cross, the better chances that the values will show the 3:1 ratio. • 18. So which would offer the better data? – An F1 generation of 100? – An F1 generation of 1000?