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Section 11-2: Probability and
Punnett Squares
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!
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?
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
Punnett Square Diagram
Dominant Alleles
T
T
t
Recessive Alleles
t
8. Homozygous
• Having two identical alleles for a particular
trait
– Ex: TT or tt
• “True-Breeding” for
• a Particular Trait
– Examples:
• Hydras
• Mushrooms
• Bacteria
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
9. Phenotype
• The physical characteristics of an organism
• In our Punnett Square example what are our
phenotypes?
– Tall
• TT or Tt
– Short
• tt
7. Genotype
• The genetic makeup of an organism
• In our Punnett Square example what would
the genotypes be?
– TT- 25%
– Tt- 50%
– tt- 25%
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.
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!
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?
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?