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# Ch.3 2 Pp

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### Ch.3 2 Pp

1. 1. Chapter 3 Section 2 Probability and Punnett Squares
2. 2. Genetics and Probability <ul><li>The likelihood that a particular event will occur is called probability </li></ul><ul><li>Ex: Flipping a coin </li></ul><ul><li>There are 2 possible outcomes…heads or tails </li></ul><ul><li>The chances, or probabilities, of either outcome are equal </li></ul>
3. 3. <ul><li>Therefore, the probability that a single coin flip will come up heads is 1 chance in 2 </li></ul><ul><li>This is 1/2 or 50% </li></ul>
4. 4. <ul><li>Let’s say you flip a coin 3 times in a row, what is the probability it will land heads up every time? </li></ul><ul><li>Think of each coin flip as an independent event </li></ul><ul><li>The probability of the coin landing heads up is 1/2…we said this already </li></ul><ul><li>The probability of flipping 3 heads in a row is: </li></ul><ul><li>1/2 x 1/2 x 1/2 = 1/8 </li></ul><ul><li>You have a 1 in 8 chance of flipping 3 heads in a row </li></ul>
5. 5. <ul><li>These same principles of probability can be used to predict the outcomes of genetic crosses! </li></ul>
6. 6. Punnett Squares <ul><li>The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a punnett square </li></ul><ul><li>Punnett squares can be used to predict and compare the genetic variations that will result from a cross. </li></ul>
7. 7. <ul><li>The parents are shown on the top and left side of the box </li></ul><ul><li>The possible gene combinations for the offspring appear in the 4 boxes that make up the square. </li></ul><ul><li>The letter in the the punnett square represent alleles. </li></ul>T T T t T t t t T t T t
8. 8. Homozygous <ul><li>Organisms that have 2 identical alleles for a particular trait are said to be HOMOZYGOUS </li></ul><ul><li>Example: TT or tt </li></ul>
9. 9. Heterozygous <ul><li>Organisms that have 2 different alleles for the same trait are referred to as HETEROZYGOUS. </li></ul><ul><li>They are hybrid for a particular trait </li></ul><ul><li>Example: Tt </li></ul>
10. 10. Phenotype vs. Genotype <ul><li>What the heck is the difference??? </li></ul><ul><li>Phenotype refers to the physical characteristics of an organism </li></ul><ul><ul><li>Tall, short, wavy, straight, etc. </li></ul></ul><ul><li>Genotype refers to the genetic makeup of an organism </li></ul><ul><ul><li>The actual alleles that give an organism its specific phenotype </li></ul></ul><ul><ul><li>Genoytype tells us if it is TT, Tt, or tt </li></ul></ul>
11. 11. <ul><li>All tall plants have the same PHENOTYPE, but they do not have the same GENOTYPE necessarily. </li></ul><ul><li>How can we explain this??? </li></ul><ul><li>Plants can be TT or Tt and still be a tall plant! </li></ul><ul><li>As long as there is a dominant allele present, the dominant form of that trait is visible! </li></ul>
12. 12. Probability & Segregation <ul><li>1/4 of the offspring have 2 alleles for tallness (TT) </li></ul><ul><li>2/4 or 1/2 have one allele for tallness and one allele for shortness (Tt) </li></ul><ul><li>1/4 of the offspring have 2 alleles for shortness (tt) </li></ul><ul><li>Overall 3/4 of the plants should be tall </li></ul><ul><li>There should be 3 tall plants and 1 short plant </li></ul><ul><li>3:1 ratio! </li></ul>T t T t T T T t T t t t
13. 13. <ul><li>For each of the 7 crosses Mendel completed and studied, 3/4 of the plants showed the trait controlled by the dominant allele </li></ul><ul><li>About 1/4 showed the trait controlled by the recessive allele. </li></ul><ul><li>Segregation did indeed occur! </li></ul><ul><li>Mendel was RIGHT! </li></ul>