1. Chapter 11: Intro to Genetics
11-2
Probability & Punnett Squares

2. Intro When Mendel performed a X he counted the offspring.
When Mendel repeated a particular X, he obtained similar results. When he X two plants that were hybrid for stem height ( Tt X Tt) about ¾ were tall & ¼ were short.
Mendel realized the principles of probability could be used to explain the result of genetic crosses.

3. Genetics & Probability
The likelihood that a particular event will occur is called?
Probability
Ex: A coin flip
There are two possibilities: heads up or tails up
The chances of either outcome are equal.
The probability that a single coin flip will land heads up is 1 chance in 2.
This is ½ , or 50%.

4. Genetics & Probability
If you flip a coin 3 times in a row, what is the probability that it will land heads up every time?
Each coin flip is an independent event. The probability of each coin landing heads up is 1/2
The probability of flipping 3 heads in a row is: ½ X ½ X ½=1/8
It is a 1 in 8 chance.
Past outcomes do not effect future outcomes.

5. Genetics & Probability
How is coin flipping relevant to genetics?
The way in which alleles segregate is completely random—like a coin flip!
For this reason, the principles of probability can be used to predict the outcome of genetic crosses.

6. Punnett Squares What is a Punnett Square?
They can be used to predict & compare the genetic variations that will result from a cross.

7. Sample Cross “T”—dominant allele for tallness.
“t”—recessive allele for shortness. T t

8. Genetic Terms
Homozygous- organisms that have two identical alleles for a trait. They are true breeding for a particular trait. Ex: TT or tt
Heterozygous-organisms that have two different alleles for a trait. They are hybrid for a particular trait. Ex: Tt
Hint: Homo- same; hetero-other

9. Genetic Terms
Phenotype-physical appearance. All tall plants have the same phenotype. They do not have the same genotype. Ex: Tall, short, green, yellow
Genotype-genetic makeup. The genotype of 1/3 of the tall plants is TT, the genotype of 2/3 of the tall plants is Tt.
The plants have the same phenotype but different genotypes.

10. Probabilities & Segregation
How can a ratio be used to describe the outcome of the sample cross.
There are 3 tall plants for every one short plant in the F2 generation.
The ratio is 3:1
For each of Mendel’s 7 crosses, about ¾ showed the trait controlled by the dominant allele. About ¼ showed the trait controlled by the recessive allele. Segregation did occur!

11. Probabilities Predict Averages
What do probabilities predict?
The average outcome of a large number of events. They do not predict the precise outcome of an individual event.
Ex: If you flip a coin twice, you are likely to get 1 head, 1 tail. You could also get 2 heads or 2 tails. To get that 50:50 ratio, you would have to flip the coin many times.

12. Probabilities Predict Averages
The same is true for genetics.
The larger the number of offspring, the closer the numbers will be to expected values.
If the F1 gen contains just 3 or 4 offspring, it may not match the predicted ratios. When the F1 gen contains hundreds or thousands of individuals, the ratios are close to expectations.