1. Probability and Punnett Squares
Chapter 11.2
Probability and Punnett Squares

2. When Mendel did his experiments he counted everything very carefully
When Mendel did his experiments he counted everything very carefully. He worked with thousands of plants. He found that the ratio of dominant to recessive offspring of 2 hybrids always came about ¾ dominant and ¼ recessive.

3. Genetics and Probability
Probability is the likelihood that a particular event will take place.
Example: flip a coin and there are 2 possible outcomes. The chance that you would get heads it 1 chance in 2 or ½.
If we flip the coin 3 times in a row every time there is a ½ chance of getting heads so to find the total probability of flipping heads 3 times in a row we would multiply ½ x ½ x ½ = 1/8
This is the same in predicting how alleles will be passed to offspring. So we can use the principles of probability to help us predict the outcomes of genetic crosses.
Genetics and Probability

4. Punnett squares are diagrams that help us predict the possible outcomes of genetic crosses.
Organisms that have 2 identical alleles for a particular trait (TT or tt) is said to be homozygous. Organisms that have different alleles for the same trait (Tt) is heterozygous.
Also a phenotype is the physical appearance of an organism.
Genotype is the genetic makeup of an organism.
You cannot always tell the genotype with the phenotype: for examples: a tall plant can have a genotype of either Tt or TT.
Punnett Squares

5. Probabilities Predict Averages
Probabilities are good at predicting the average outcome of a large number of events but cannot predict the precise outcome of an individual event.
It gives you the chances of getting a certain genotype but not exact numbers.
Again, don’t worry we will go over this in class!
Probabilities Predict Averages