1. 11-2 Probability and Punnett Squares

2. Genetics and Probability
Genetics is based on probability
probability: the likelihood that a particular event will occur
the probability of a particular event occurring is divided by all the possible outcomes
past outcomes do not affect future ones
principles of probability can be used to predict outcomes of genetic crosses

3. Probability of getting heads on a coin in one flip is 1 out of 2 or 1/2 or .5 or 50%
What if you want to know the chance of getting two coins to both be tails?
You simply multiply each of the coins separate probabilities together
½ x ½ = ¼
Called the Rule of Multiplication

4. Probability ranges from 0 to 1
0 = the event will not occur (0 % chance)
1 = the event will occur (100 % chance)
There can be fractions in between
All the possible outcome probabilities must add up to one

5. Probabilities Predict Averages
in order to get the predicted average for a particular event, the event would have to occur many times
in a genetics cross, you need many offspring in a generation to come close to the predicted ratio

6. Genetic Terminology
Punnett squares: a grid used for determining the genetic combinations that would result from a genetic cross
These predict the possible outcome percentages of the offspring

7. parents
gametes

8. predicted offspring

9. All traits have a 3:1 ratio in F2 generation
Parents
Long stems  short stems
Red flowers  white flowers
Green pods  yellow pods
Round seeds  wrinkled seeds
Yellow seeds  green seeds
First Generation
All long
All red
All green
All round
All yellow
Second Generation
787 long: 277 short
705 red: 224 white
428 green: 152 yellow
5474 round: 1850 wrinkled
6022 yellow: 2001 green
Results
All traits have a 3:1 ratio in F2 generation