1. Section 11-2: Probability and Punnett Squares

2. 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!

3. 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?

4. 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

5. Punnett Square Diagram
Dominant Alleles
Recessive Alleles T t

6. 8. Homozygous Having two identical alleles for a particular trait
Ex: TT or tt
“True-Breeding” for
a Particular Trait
Examples:
Hydras
Mushrooms
Bacteria

7. 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

8. 9. Phenotype The physical characteristics of an organism
In our Punnett Square example what are our phenotypes?
Tall
TT or Tt
Short tt

9. 7. Genotype The genetic makeup of an organism
In our Punnett Square example what would the genotypes be?
TT- 25%
Tt- 50%
tt- 25%

10. 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.

11. 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!

12. 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?

13. 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?