1. Chapter 11 Introduction to Genetics
11-2 Probability and Punnett Squares
Objectives:
Explain how geneticists use the principles of probability.
Describe how geneticists use Punnett squares.

2. Genetics and Probability

3. Checkpoint

4. Punnett Squares
The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.
This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

5. Punnett squares can be used to predict and compare the genetic variations that will result from a cross.
Organisms that have two identical alleles for a particular trait – TT or tt in this example – are said to be homozygous (true-breeding for a particular trait).
Organisms that have two different alleles for the same trait – Tt in this example – are said to be heterozygous (hybrid for a particular trait).

6. All of the tall plants have the same phenotype, or physical characteristics.
They do not, however, have the same genotype, or genetic makeup.
The genotype of one third of the tall plants is TT, while the genotype of two thirds of the tall plants is Tt.

7. Probability and Segregation

8. Probabilities Predict Averages
Probabilities predict the average outcome of a large number of events.
However, probability cannot predict the precise outcome of an individual event.

9. 11-2 Section Assessment
How are the principles of probability used to predict the outcomes of genetic crosses?
How are Punnett squares used?
What is probability?
Define genotype and phenotype?