section 11-2: probability and punnett squares. 1. probability the likelihood that a particular event...
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Section 11-2: Probability and Punnett Squares
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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!
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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?
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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
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Punnett Square Diagram
T t
T
t
Dominant Alleles Recessive Alleles
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8. Homozygous
• Having two identical alleles for a particular trait– Ex: TT or tt
• “True-Breeding” for• a Particular Trait– Examples:• Hydras• Mushrooms• Bacteria
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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
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9. Phenotype
• The physical characteristics of an organism• In our Punnett Square example what are our
phenotypes?– Tall• TT or Tt
– Short• tt
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7. Genotype
• The genetic makeup of an organism• In our Punnett Square example what would
the genotypes be?– TT- 25%– Tt- 50% – tt- 25%
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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.
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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!
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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?
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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?