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

T t

T

t

Dominant Alleles Recessive Alleles

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?